Lymphokine regulation of CD45R expression on human T cell clones

Characterization of CD34+ Cells from Patients with Acute Myeloid Leukemia (AML) and Myelodysplastic Syndromes (MDS) Using a t-Distributed Stochastic Neighbor Embedding (t-SNE) Protocol

Using multi-color flow cytometry analysis, we studied the immunophenotypical differences between leukemic cells from patients with AML/MDS and hematopoietic stem and progenitor cells (HSPCs) from patients in complete remission (CR) following their successful treatment. The panel of markers included CD34, CD38, CD45RA, CD123 as representatives for a hierarchical hematopoietic stem and progenitor cell (HSPC) classification as well as programmed death ligand 1 (PD-L1). Rather than restricting the evaluation on a 2- or 3-dimensional analysis, we applied a t-distributed stochastic neighbor embedding (t-SNE) approach to obtain deeper insight and segregation between leukemic cells and normal HPSCs. For that purpose, we created a t-SNE map, which resulted in the visualization of 27 cell clusters based on their similarity concerning the composition and intensity of antigen expression. Two of these clusters were "leukemia-related" containing a great proportion of CD34+/CD38- hematopoietic stem cells (HSCs) or CD34+ cells with a strong co-expression of CD45RA/CD123, respectively. CD34+ cells within the latter cluster were also highly positive for PD-L1 reflecting their immunosuppressive capacity. Beyond this proof of principle study, the inclusion of additional markers will be helpful to refine the differentiation between normal HSPCs and leukemic cells, particularly in the context of minimal disease detection and antigen-targeted therapeutic interventions. Furthermore, we suggest a protocol for the assignment of new cell ensembles in quantitative terms, via a numerical value, the Pearson coefficient, based on a similarity comparison of the t-SNE pattern with a reference.

CD45RA, CD8β, and IFNγ Are Potential Immune Biomarkers of Human Cognitive Function

There is increasing evidence that in humans the adaptive immunological system can influence cognitive functions of the brain. We have undertaken a comprehensive immunological analysis of lymphocyte and monocyte populations as well as of HLA molecules expression in a cohort of elderly volunteers (age range, 64–101) differing in their cognitive status. Hereby, we report on the identification of a novel signature in cognitively impaired elderly characterized by: (1) elevated percentages of CD8+ T effector-memory cells expressing high levels of the CD45RA phosphate receptor (Temra^hi); (2) high percentages of CD8+ T cells expressing high levels of the CD8β chain (CD8β^hi); (3) augmented production of IFNγ by in vitro activated CD4+ T cells. Noteworthy, CD3+CD8+ Temra^hi and CD3+CD8β^hi cells were associated with impaired cognition. Cytomegalovirus seroprevalence showed that all volunteers studied but one were CMV positive. Finally, we show that some of these phenotypic and functional features are associated with an increased frequency of the HLA-B8 serotype, which belongs to the ancestral haplotype HLA-A1, Cw7, B8, DR3, DQ2, among cognitively impaired volunteers. To our knowledge, this is the first proof in humans linking the amount of cell surface CD45RA and CD8β chain expressed by CD8+ Temra cells, and the amount of IFNγ produced by in vitro activated CD4+ T cells, with impaired cognitive function in the elderly.

3A4, a new potential target for B and myeloid lineage leukemias

Antibody-targeting therapy has drawn great interests to the hematologists and oncologists. Many antibodies have been studied for their potential targeting for hematopoietic malignancies. A few have been proved to be very effective for patients with these diseases. However, more antibodies are needed for clinical use. CD45 and its isoforms may convey clinical potential in terms of targeting therapy. Zhejiang Children's Hospital (ZCH)-6-3A4 (3A4), a novel antibody that can recognize an isoform of CD45 has been found to react with restricted cell components in hematopoietic system, which may have the potential for targeting therapy. Herein, we conducted an in vitro study of our newly prepared antibody 3A4 using various cellular and immunocytological methods. The results showed that the antibody 3A4 (murine IgG1κ) was a new clone of anti-CD45RA. It could block the binding to an epitope of CD45RA recognized by a standard anti-CD45RA antibody (Clone name L48). The reactivity of the 3A4 to both fresh leukemia cells from patients and well-defined leukemia cell lines was largely similar to those of L48, but the former recognized more leukemia cells than the latter. Cytometric analysis after papain treatment showed that the internalization rate of the 3A4 antibody to the target cells was as high as 71.3% after incubation at 37°C for 4 h, which was significantly higher than that of L48 (20.4%). The norcantharidin (NCTD)-conjugated immunotoxin (NCTD-3A4) was generated using an active ester method. The targeting inhibition rate on KG1a was as high as 61.10% after 96 h incubation in a dose-dependent manner, which was significantly higher than that (3.56%, P < 0.01) with 3A4-negative Nalm-6 cells. In conclusion, our new anti-CD45RA antibody 3A4 is probably a new target molecule of leukemia cells and holds a targeting therapeutic potential for hematopoietic malignancies, which warrants further development of this agent.

FOXP3+ regulatory T cells in the human immune system

Forkhead box P3 (FOXP3)(+) regulatory T (T(Reg)) cells are potent mediators of dominant self tolerance in the periphery. But confusion as to the identity, stability and suppressive function of human T(Reg) cells has, to date, impeded the general therapeutic use of these cells. Recent studies have suggested that human T(Reg) cells are functionally and phenotypically diverse. Here we discuss recent findings regarding human T(Reg) cells, including the ontogeny and development of T(Reg) cell subsets that have naive or memory phenotypes, the unique mechanisms of suppression mediated by T(Reg) cell subsets and factors that regulate T(Reg) cell lineage commitment. We discuss future studies that are needed for the successful therapeutic use of human T(Reg) cells.

Allelic variant in CTLA4 alters T cell phosphorylation patterns

Little is known regarding the functional effects of common autoimmune susceptibility variants on human immune cells. The SNP CT60 (rs3087243; A/G) located in the 3' UTR of the CTLA4 gene has been associated with autoimmune diseases. We examined a cohort of healthy individuals stratified by genotypes at CTLA4 to gain insight into the functional effects of allelic variation on T cell signaling. Using phospho-site-specific mAbs, we tested the hypothesis that the CT60 genotype at CTLA4 is associated with altered T cell antigen receptor (TCR) signaling in naive and/or memory T cells. By normalizing for the extent of the initial TCR signaling event at CD3zeta, we observed that the relative responsiveness to TCR stimulation as assessed by phosphorylation levels of downstream signaling molecules was altered in naive (CD4(+)CD45RA(high)) and memory (CD4(+)CD45RA(low)) T cells obtained from individuals with the disease-susceptibility allele at CTLA4. Thus, allelic variation associated with autoimmune disease can alter the signaling threshold of CD4(+) T cells. These experiments provide a rational approach for the dissection of T cell-susceptibility genes in autoimmune diseases.

Putative role for interleukin-7 in the maintenance of the recirculating naive CD4+ T-cell pool

The capacity of the immune system to respond efficiently to new antigens depends upon a continuous source of naive CD4+ T cells. Such cells exit from the thymus and join the recirculated T-cell pool. Factors present at the sites of naive CD4+ T-cell circulation must be responsible for their survival, since upon removal from their host, naive CD4+ T cells die. However, such factors remain unknown. The presence of the cytokine interleukin-7 (IL-7) in secondary lymphoid organs and the continuous expression of its receptor on naive CD4+ T cells prompted us to examine the possibility that IL-7 might be a survival factor for naive CD4+ T cells. Using naive CD4+ T cells isolated from cord blood we show that IL-7, but not IL-2, can maintain naive CD4+ T-cell viability in vitro for at least 15 days. In addition, we find that IL-7 can induce modest proliferation of naive CD4+ T cells without affecting either their cell surface phenotype or their ability to respond to antigenic stimulation. We also find that after anti-CD3 stimulation, naive CD4+ T cells lose that ability to respond to IL-7. However, if cells are primed with IL-7 prior to antigenic stimulation, their proliferative responses are enhanced. Together, these data suggest a novel and important role for IL-7 in the maintenance and maturation of naive CD4+ T cells, ensuring that they can respond maximally when they first meet antigen in secondary lymphoid tissue.

Rapid conversion of naive to effector T cell function counteracts diminished primary human newborn T cell responses

The reduced incidence of graft versus host disease following the use of human cord blood as a source of stem cells for bone marrow reconstitution challenges our understanding of the immunocompetence of newborn T cells. Newborn CD4+ T cells express mainly the CD45RA phenotype and have been considered to respond comparably to adult CD4+ T cells exhibiting the CD45RA phenotype. We compared the in vitro kinetics of phenotypic conversion of newborn and adult CD4+CD45RA+ T cells to CD4+CD45RO+ T cells. The cytokine profile and B cell helper activity of the converted CD4+CD45RO+ T cell population were also determined. Newborn CD4+CD45RA+ T cells were converted to CD4+CD45RO+ with significantly faster time kinetics than adult CD4+CD45RA+ T cells, following either phytohaemagglutinin (PHA) or anti-CD2 activation. Freshly purified newborn naive T cells did not produce IL-2, IL-4 or interferon-gamma (IFN-gamma) following stimulation, whereas adult naive T cells secreted IL-2 and adult-derived CD4+CD45RO+ T cells secreted all three cytokines under the same stimulatory conditions. However, newborn and adult CD4+CD45RA+ T cells, following primary stimulation and maturation in vitro, acquired the ability to secrete a Th1-type cytokine profile of IL-2 and IFN-gamma after secondary stimulation. Newborn CD4+ naive T cells that acquired the CD45RO phenotype in vitro also gained B cell helper activity equivalent to that of adult in vitro matured CD4+ naive T cells. These findings suggest that newborn and adult CD4+CD45RA+ T cell subsets are differentially responsive to various stimuli. They show that newborn CD4+CD45RA+ naive T cells can transform more quickly than their adult counterparts into functionally equivalent CD4+CD45RO+ T cells, a process that may be important to counteract the immature immune environment which exists in the newborn.

Adoptively transferable tolerance induced by CD45RB monoclonal antibody

The phenomenon of rejection remains the most serious problem in transplantation. The ultimate goal in transplant immunology is to develop therapeutic strategies that lead to tolerance. It has been shown that two injections of a monoclonal antibody to CD45RB leads to indefinite acceptance of renal allografts in mice. Moreover, the CD45RB monoclonal antibody reverses acute rejection and still induces tolerance. The purpose of this study was to assess mechanisms that could underlie this therapeutic benefit. It was shown that splenic lymphocytes from tolerant animals augmented proliferation in allogeneic mixed lymphocyte reactions against donor alloantigens, and the serum of tolerant mice contained donor-specific antibodies, mainly of the IgG1 isotype, suggesting the presence of TH2 cytokines. Tolerance could not be broken by interleukin-2 infusion, but tolerance could be adoptively transferred by transfusion of tolerant mouse CD4+ splenic lymphocytes into naive allografted animals. These data suggest that an active immunoregulatory mechanism is partly responsible for the therapeutic effect. CD45RB-directed therapy may find clinical application in organ transplantation in human patients.

Direct Analysis of Viral-Specific CD8+ T Cells with Soluble HLA-A2/Tax11-19 Tetramer Complexes in Patients with Human T Cell Lymphotropic Virus-Associated Myelopathy

Human T cell lymphotropic virus-I (HTLV-I)-associated myelopathy is a slowly progressive neurologic disease characterized by inflammatory infiltrates in the central nervous system accompanied by clonal expansion of HTLV-I-reactive CD8+ T-cells. In patients carrying the HLA-A2 allele, the immune response is primarily directed to the Tax11-19 peptide. The frequency, activation state, and TCR usage of HLA-A2/Tax11-19 binding T cells in patients with HTLV-I-associated myelopathy was determined using MHC class I tetramers loaded with the Tax11-19 peptide. Circulating Tax11-19-reactive T cells were found at very high frequencies, approaching 1:10 circulating CD8+ T cells. T cells binding HLA-A2/Tax11-19 consisted of heterogeneous populations expressing different chemokine receptors and the IL-2R β-chain but not the IL-2R α-chain. Additionally, Tax11-19-reactive CD8+ T cells used one predominant TCR Vβ-chain for the recognition of the HLA-A2/Tax11-19 complex. These data provide direct evidence for high frequencies of circulating Tax11-19-reactive CD8+ T cells in patients with HTLV-I-associated myelopathy.

Differentiation of T lymphocytes in the human adenoid as measured by the expression of CD45 isoforms

Encounter of antigen by T lymphocyte on antigen-presenting cells results in changes in the expression of several cell surface molecules, including the abundant cell surface glycoprotein CD45. We have characterized the expression of the CD45 isoforms CD45RA and CD45RO in CD4+ and CD8+ T lymphocytes in the adenoids and peripheral blood of young children. We found that the relative proportions of CD45RA-,CD45RO+ antigen-experienced T cells was higher in the adenoids than in peripheral blood, and that the proportion of naive or resting CD45RA+,CD45RO- T cells was lower in the adenoids than in peripheral blood. The frequency of bright double-positive CD45RA+,CD45RO+ T cells, which represent cells in transition from the CD45RA+ to CD45RO+ phenotype, was higher in the adenoids than in peripheral blood. The frequency of another double-positive cell population, but with unknown ontogeny, expressing both CD45RA and CD45RO at a low level, was higher in peripheral blood than in adenoidal T cells. It was found that the frequency of adenoidal antigen-experienced CD45RA-,CD45RO+ T lymphocytes increased with increasing age of the child. These results are consistent with the model that the adenoids serve as a site for conversion of CD45RA+ to CD45RO+ T lymphocytes, and that the maturation of the immune system in young children is associated with phenotypic changes in T lymphocytes residing in secondary lymphoid organs.

Differential Susceptibility to Activation-Induced Apoptosis Among Peripheral Th1 Subsets: Correlation with Bcl-2 Expression and Consequences for AIDS Pathogenesis

It has been proposed that HIV infection is associated with an imbalance in Th1 and Th2 subsets. Recent reports indicate that Th1 and Th2 effectors differ in their susceptibility to activation-induced apoptosis. To determine whether increased T cell apoptosis in HIV-infected patients contributes to alterations in cytokine synthesis, we performed single-cell analysis of type 1 and type 2 cytokine production by CD4 and CD8 T cells, simultaneously with detection of apoptosis. We demonstrate that a differential alteration in representation of Th1 subsets, rather than commitment of T cells to secrete Th2 cytokines, occurs throughout HIV infection. A significant decrease in the number of IL-2- or TNF-α-producing T cells was observed, whereas those producing IFN-γ remained preserved. Furthermore, there is a gradient of susceptibility to activation-induced apoptosis (IL-2 &lt; IFN-γ &lt; TNF-α) among the different Th1 subsets. This gradient was detected in both CD4 and CD8 subsets, as well as in control donors and HIV-infected patients, in whom the susceptibility to apoptosis of IL-2 and IFN-γ producers was increased compared with controls. This differential intrinsic apoptosis susceptibility of Th1 effectors was found to be tightly regulated by Bcl-2 expression. In HIV-infected persons, disappearance of IL-2-producing T cells was a good indicator of disease progression and was correlated with the progressive shrinkage of the CD4+CD45RA+ T cell compartment and a gradual increased susceptibility to activation-induced apoptosis of the IL-2-producing subset. This close relationship between the CD45RA/CD45R0 ratio, the level of type 1 cytokine production, and susceptibility to apoptosis should be considered in HIV-infected patients under antiviral or immune-based therapies.

Identical expression of CD45R isoforms by CD45RC+ 'revertant' memory and CD45RC+ naive CD4 T cells

Naive and memory CD4 T cells are frequently defined by exon-specific monoclonal antibodies (mAb) which stain (or not) high- or low-molecular-weight (MW) isoforms of the leucocyte common antigen CD45. The link between isoform and the naive/memory designation is complicated by the fact that CD4 T cells with a 'memory' phenotype (CD45RA-, RB-, RC-, or CD45RO+) may revert ('revertants') and re-express the high mw isoform (CD45RA+, RB+, RC+). Isoform expression also changes during normal T-cell development. Furthermore, the picture may be incomplete since an exon-specific mAb will not detect all possible isoforms on a cell. We have used molecular techniques to determine whether revertant CD4 memory T cells were different from naive T cells with respect to CD45R isoform expression. Using the anti-CD45RC mAb OX22 to purify rat lymphocyte subsets, CD45R isoform expression was examined at the mRNA level in CD4 T cells at different stages of development and compared with that of B cells and unseparated lymphocytes. B cells contained abundant message for the highest MW 3-exon isoform ABC, the 2-exon isoforms AB and BC, and the null isoform O. Both immature CD45RC- (i.e. CD4+8- 'single positive' thymocytes, and peripheral Thy-1+ recent thymic emigrants) and mature CD45RC- 'antigen-experienced' CD4 T cells had message for single-exons B, possibly C and for the O exon. In contrast, CD45RC+ CD4 T cells contained mRNA coding for ABC (low level), AB, BC, B, C (low level) and O (low level). Importantly, there was no difference between CD45RC+ T cells that had not seen antigen ('truly native') and CD45RC+ antigen-experienced revertant memory T cells. This observation has implications for understanding long-term immunological memory.

CD45RA-expressing memory/effector Th cells committed to production of interferon-gamma lack expression of CD31

It has been considered before that human naive and memory/effector CD4+ T-cells cannot be subdivided solely according to the differential expression of CD45 isoforms. By the lack of expression of CD31 we have identified a subset of CD4+ CD45RA+ CD31- cells which show distinct features of antigen-experienced Th1 cells. Short term stimulation of highly purified human peripheral blood CD4+ T-cells with PMA/ionomycin, followed by the cytometric analysis of intracellular cytokines, showed that a minor subpopulation of CD4+ CD45RA+ CD45RO- cells is able to produce interferon-gamma (IFN-gamma) rapidly, a characteristic of antigen-experienced Th1 cells. Whereas among CD45RA+ CD4+ T-cells both CD31+ and CD31- subsets produce interleukin-2 (IL-2) upon PMA/ionomycin stimulation, only the CD31- subpopulation is able to produce IFN-gamma. Thus, our phenotypic and functional characterization of CD45RA+ CD45RO- Th cells shows that CD45RA+ CD45RO- cells do not represent a homogeneous population of antigen-unexperienced, naive T-cells. We speculate that a certain subset of human CD4+, CD45RO+ memory T-cells reverts to expression of the CD45RA isoform, and that this subset can be identified by the lack of CD31 expression.

CD45RC isoforms define two types of CD4 memory T cells, one of which depends on persisting antigen

The cellular basis of immunological memory remains a controversial area with respect to the identity of memory T cells and the role of persisting antigen. CD4 T cells are phenotypically divided by the expression of high and low molecular weight isoforms of CD45, surface markers that are frequently used to identify "naive" (CD45Rhigh) and "memory" (CD45Rlow) subsets. The latter subset responds rapidly in antigen recall assays but paradoxically has a short life span, a property that is difficult to reconcile with long-term memory. The present study examines these issues using a DTH (delayed-type hypersensitivity) model in which contact sensitivity to dinitrochlorobenzene (DNCB) was transferred to athymic nude rats by recirculating CD4 T cell subsets defined in the rat by the anti-CD45RC mAb OX22. As expected, CD45RC+ (but not RC-) CD4 T cells from normal unprimed rats transferred a DNCB-specific DTH response, whereas, 4 d after sensitization the CD45RC- (memory) subset alone contained the DNCB reactivity. However, when donor cells were collected from thymectomized rats sensitized two mo earlier, DNCB-specific responses were transferred by both CD45RC- and RC+ subsets suggesting that many of the latter had developed from cells with a memory phenotype. This was confirmed when CD45RC CD4 T cells from 4-d primed rats were parked in intermediate nude recipients and recovered 2 mo later. DNCB-specific activity was now found wholly within the CD45RC+ "revertant" subset; the CD45RC-CD4 T cell population was devoid of activity. Importantly, we found that the total switch-back from CD45RC- to RC+ could be prevented, apparently by persisting antigen. The results indicate that there are two functionally distinct categories of memory T cells: one, a short-lived CD45Rlow type which orchestrates the rapid kinetics, the other, a longer-lived CD45Rhigh revertant which ensures that immunological memory endures.

CD45 isoforms on human CD4+ T-cell subsets

We have examined the level of surface expression and functional properties of leukocyte function associated antigen-1, very late antigen-4, and CD45 isoforms on a panel of human CD4+ T-cell clones representative of TH0, TH1, and TH2 cells. There were no qualitative differences in the expression of these antigens among the three types of CD4+ T-cell clones. However, CD45RB was the only CD45 isoform that provided a costimulatory signal in a solid-phase antibody-induced cellular proliferation assay. Additionally, the antigen-induced proliferative response of T-cell clones was inhibited by soluble anti-CD45RO and anti-CD45RB antibodies. Our results suggest that CD45 isoforms differentially provide costimulatory signals to T cells. However, the ability of each CD45 isoform to provide a costimulatory signal does not differ among the TH0, TH1, or TH2 T-cell populations.

CD31 (PECAM-1) is a differentiation antigen lost during human CD4 T-cell maturation into Th1 or Th2 effector cells

The CD31 antigen (PECAM-1) has been reported to be a stable marker for a human CD4 T-cell subpopulation unable to produce interleukin-4 (IL-4). We show here that CD31 expression is not stable inasmuch as CD4 T-cell lines and clones derived from cell-sorted neonatal CD31+ cells lose CD31 upon repetitive cycles of stimulation and IL-2 expansion. Moreover, various cytokines (IL-1 alpha, IL-4, IL-6, transforming growth factor-beta) fail to reinduce CD31 on CD31- clones. Whereas all CD31+ CD4 T cells rapidly express high levels of the CD45RO antigen and down-regulate the L-selectin antigen after priming, CD31 disappears more slowly because only part of the cells lose CD31 expression upon each cycle of stimulation. Loss of CD31 reflects a functional maturation of CD45RO+ cells since, in a system which favours the development of Th2 effectors, IL-4 is produced by CD31- but not CD31+ effector T cells, whereas interferon-gamma is produced by both types of cells. However, CD31 is not a Th1 marker since it is not expressed on several Th1 antigen-specific clones. We conclude that CD31 is a maturation marker expressed on the great majority of naive CD45RO- CD4 T cells and on a subset of CD45RO+ CD4 T cells that are at an intermediate stage of maturation.

CD45 isoforms expression on CD4+ and CD8+ T cells throughout life, from newborns to centenarians: implications for T cell memory

CD4+ and CD8+ peripheral blood T lymphocytes show mutually exclusive expression of CD45RA or CD45R0, two isoforms of the common leukocyte antigen that seem to recognize so-called virgin/unprimed and memory/activated T cells. The expression of these isoforms has been studied by three colour cytofluorimetric analysis on CD4+ or CD8+ peripheral blood CD3+ cells from 22 healthy centenarians, analyzed in a context of 202 healthy donors 0-110 years old. An age-related unbalance of virgin and memory cells was found between CD4+ and CD8+ subsets. As expected, at birgh 95-99% of the CD3+ lymphocytes expressed the CD45RA isoform. A rapid increase of CD45R0+ cells was observed in the first 2-3 decades of life, this phenomenon being much more pronounced on CD4+ cells. Subsequently, the increase of the 'memory' compartment was much less rapid, so that in centenarians a consistent reservoire of CD45RA+ among CD4+ cells was still present (about 20%). In these exceptional individuals the percentage of CD45RA+ cells among CD8+ T lymphocytes was even higher (about 50%), and only slightly lower than that of young donors (about 55-60%). Thus, the main changes occurred at a different rate in CD4+ (about 20%). In these exceptional individuals the percentage of CD45RA+ cells among CD8+ T lymphocytes was even higher (about 50%), and only slightly lower than that of young donors (about 55-60%). Thus, the main changes occurred at a different rate in CD4+ and in CD8+ T cells, at an age of between 0 and 30 years, when the thymus is still functionally active. Interestingly, no difference in the usage of CD45 isoforms was observed within T cells bearing four different V beta-T cell receptor (TCR). The significance of this age-related unbalance is unknown. However, the presence of a great number of CD45RA+ T lymphocytes within the CD4+ and the CD8+ T cell subsets even in the peripheral blood of centenarians poses the problem of their origin (thymus? extrathymic sites?), of their functional role and of their lifespan. Moreover, the data on centenarians suggest that they may represent a very selected population where a slowing of immunosenescence occurs.

Cellular immunity in autoimmune thyroid disease

Autoimmune thyroid disease occurs in a genetically susceptible patient after triggering events including bacterial and viral infections, environmental insults, drugs or hormones. These triggering events may break the tolerance to self-antigen, leading to emergence of autoreactive T cells. One or more T cell clones that recognize the self-antigen is(are) assumed to be involved in initiating autoimmune processes. Following this, T cell clones expand and migrate from the peripheral blood into the thyroid gland. Migration of mononuclear cells is controlled by inflammatory cytokines and adhesion molecules. Intrathyroidal T cells may interact with dendritic-like cells, thyrocytes expressed with HLA-DR antigens, B cells and extracellular matrix, resulting in the proliferation of T cells, production of cytokines and autoantibodies. These interactions are also regulated by inflammatory cytokines and adhesion molecules. When the initial immune response is completed, a secondary immune response ensues, that may be of considerable complexity involving reaction of infiltrating T cells to a variety of tissue-specific and tissue-non-specific antigens. These immune responses may contribute to the recurring immunologic activity and maintenance of autoantibody overproduction.

Evidence for an immune response in major depression: a review and hypothesis

1. This paper reviews recent findings on cellular and humoral immunity and inflammatory markers in depression. 2. It is shown that major depression may be accompanied by systemic immune activation or an inflammatory response with involvement of phagocytic (monocytes, neutrophils) cells, T cell activation, B cell proliferation, an "acute" phase response with increased plasma levels of positive and decreased levels of negative acute phase proteins, higher autoantibody (antinuclear, antiphospholipid) titers, increased prostaglandin secretion, disorders in exopeptidase enzymes, such as dipeptidyl peptidase IV, and increased production of interleukin (IL)-1 beta and IL-6 by peripheral blood mononuclear cells. 3. It is hypothesized that increased monocytic production of interleukins (Il-1 beta and Il-6) in severe depression may constitute key phenomena underlying the various aspects of the immune and "acute" phase response, while contributing to hypothalamic-pituitary-adrenal-axis hyperactivity, disorders in serotonin metabolism, and to the vegetative symptoms (i.e. the sickness behavior) of severe depression.

Membrane CD45R isoform exchange on CD4 T cells is rapid, frequent and dynamic in vivo

CD4 T cells bearing high (240-190 kDa) and low (180 kDa) molecular mass isoforms of the leukocyte common antigen CD45 define functionally distinct subsets which have been equated with naive and memory T cells. In the rat, CD4 T cells expressing a high molecular mass isoform [identified by monoclonal antibody MRC-OX22 (anti-CD45RC)] exchange this for the 180 kDa molecule (CD45RC-) when stimulated by antigen. Here we show, by transferring mature allotype-marked CD45RC- CD4 T cells (depleted of immature Thy-1+ CD45RC- recent thymic emigrants) into normal euthymic recipients, that many T cells re-express the high molecular mass isoform in less than 6 h. By 24 h, 30-60% of CD45RC- CD4 T cells became CD45RC+; within a week the entire cohort appeared to exchange the low for the high molecular mass isoform. Isoform exchange was dynamic and many CD4 T cells returned once again to the CD45RC- state. CD45RC- CD4 T cells declined in number more rapidly than the CD45RC+ subset after transfer. The results suggest that CD45R isoforms distinguish between resting T cells (CD45RC+) and those which have encountered antigen in the recent past. CD45R isoforms would appear to be unsuitable markers of naive and memory T cells.

CD26 surface antigen expression on peripheral blood T lymphocytes from children with Down's syndrome (trisomy 21)

A phenotypical analysis carried out by two-colour flow cytometry showed that the proportion of circulating CD4+ T lymphocytes co-expressing the membrane-associated ectoenzyme dipeptidyl peptidase IV (CD26 antigen), a functional collagen receptor involved in T-cell triggering through its interaction with the CD45 protein tyrosine phosphatase, was significantly lower in 28 children with non-translocated trisomy 21 (Down's syndrome) (DS) than that calculated in the bloodstream of 27 age- and sex-matched healthy controls. Agonist anti-CD26 monoclonal antibodies (MoAbs), such as anti-1F7, not only modulate the surface expression of this molecule, but also enhance the proliferative activity of normal human T cells via the CD3- and CD2-mediated activation pathways. T-lymphocyte proliferation induced by antigen or polyclonal T-cell activators, including anti-CD3 or -CD2 MoAbs, is severely impaired in DS. Although the physiological ligand of CD26 surface structure is unknown, the fact that CD4+ T lymphocytes found in the blood of trisomic subjects are mostly CD26- (anti-1F7-) suggests that their faulty mitogenic response may be due to phenotypical and, perhaps, strictly correlated functional abnormalities.

Activation signal induces the expression of B cell-specific CD45R epitope (6B2) on murine T cells

T lymphocytes express multiple forms of the leukocyte-common antigen CD45, transcribed by alternative usage of leukocyte-common antigen exon 4-6. The various isoforms of CD45R expressed differentially on T cells are involved in different stages of development and activation. The monoclonal antibody (MoAb) RA3-6B2 is established as a B cell-type isoform (B220)-specific marker. However, it reacts with certain activated T cells although the relationship between 6B2 expression and T-cell activation is unclear. We have examined the 6B2 expression on activated T cells and found that concanavalin A, anti-CD3 antibody and staphylococcal enterotoxin B (SEB) induced 6B2 expression on T cells. The expression was found on both CD4+ and CD8+ T cells and also was induced by SEB in vivo predominantly on CD8+ T cells. The 6B2+ T cells are IL-2R+ and blasted cells according to flow cytometry analysis. Therefore, the 6B2+ T cells are supposed to be in an activated stage. Enzymatic analysis demonstrated that trypsin treatment decreased the 6B2 expression, whereas neuraminidase increased the intensity on activated T cells. Neither endo-D or endo-H have any effect on the expression and there are no differences, in the results of immunoprecipitation and RT-PCR analysis, between control T cells and activated T cells. Taken together, the 6B2 epitope is presumed to be the product of CD45R modification and is expressed on activated T cells. These results illustrate a novel classification of a T-cell subpopulation bearing a 6B2 epitope.

Interleukin 2 induces the expression of CD45RO and the memory phenotype by CD45RA+ peripheral blood lymphocytes

The CD45RA and CD45RO isoforms of the leukocyte common antigen identify functionally distinct "naive" and "memory" T cell subsets. While antigenic and mitogenic stimuli are known to initiate transition from the naive to memory state, little is known about the role of cytokines in this process. This report demonstrates that in vitro exposure of purified CD45RA+/CD45RO- peripheral blood lymphocytes (PBL) to interleukin 2 (IL-2) promotes their conversion to the CD45RA-/CD45RO+ phenotype. Conversion to CD45RO occurs for both the CD3+ and CD3-/CD56+ lymphocyte subsets, but occurs more rapidly, and at lower IL-2 concentrations, in the CD3-/CD56+ population. Expression of CD45RO was observed only in response to IL-2 and was not observed during long-term culture in IL-4, IL-6, or IL-7. We also examined the effect of IL-2 on the expression of adhesion molecules by T cells. The expression of CD2, CD11a, and CDw29 increased, and expression of Leu-8 (LAM-1) decreased, on cultured CD45RA+/CD45RO- cells after they converted to expression of CD45RO. In contrast, lymphocytes that remained CD45RA+/CD45RO- after 10 d in culture exhibited no change from their baseline adhesion molecule profile. Finally, to test the role of endogenous IL-2 during T cell activation we stimulated CD45RA+/CD45RO- PBL with immobilized anti-CD3 in the presence of neutralizing anti-IL-2 antibody and/or cyclosporin A. Both agents significantly reduced the expression of CD45RO and the effect of cyclosporin A was reversed by exogenous IL-2. We conclude that IL-2 promotes CD45RA+ cells to express the memory phenotype and is a mediator of CD45RO expression after stimulation of the T cell receptor/CD3 complex.

Increased frequency of interleukin 2-responsive T cells specific for myelin basic protein and proteolipid protein in peripheral blood and cerebrospinal fluid of patients with multiple sclerosis

Equal numbers of CD4+ T cells recognizing myelin basic protein (MBP) and proteolipid protein (PLP) are found in the circulation of normal individuals and multiple sclerosis (MS) patients. We hypothesized that if myelin-reactive T cells are critical for the pathogenesis of MS, they would exist in a different state of activation as compared with myelin-reactive T cells cloned from the blood of normal individuals. This was investigated in a total of 62 subjects with definitive MS. While there were no differences in the frequencies of MBP- and PLP-reactive T cells after primary antigen stimulation, the frequency of MBP or PLP but not tetanus toxoid-reactive T cells generated after primary recombinant interleukin (rIL-2) stimulation was significantly higher in MS patients as compared with control individuals. Primary rIL-2-stimulated MBP-reactive T cell lines were CD4+ and recognized MBP epitopes 84-102 and 143-168 similar to MBP-reactive T cell lines generated with primary MBP stimulation. In the cerebrospinal fluid (CSF) of MS patients, MBP-reactive T cells generated with primary rIL-2 stimulation accounted for 7% of the IL-2-responsive cells, greater than 10-fold higher than paired blood samples, and these T cells also selectively recognized MBP peptides 84-102 and 143-168. In striking contrast, MBP-reactive T cells were not detected in CSF obtained from patients with other neurologic diseases. These results provide definitive in vitro evidence of an absolute difference in the activation state of myelin-reactive T cells in the central nervous system of patients with MS and provide evidence of a pathogenic role of autoreactive T cells in the disease.

Fine T-cell subsets in alcoholics as determined by the expression of L-selectin, leukocyte common antigen, and beta-integrin

Alcoholics admitted to the hospital solely for detoxication have been studied by flow cytometry to evaluate changes in the surface markers of peripheral blood leukocytes. As we have shown previously, such patients have an elevated percentage of CD8hi lymphocytes that are HLA DR+; we now demonstrate that they also have striking alterations in the quantitative relationships of the fine T-cell subsets. Both CD4+ and CD8hi lymphocytes have a sharply reduced percentage of the L-selectin+ CD45RA+ subset, increased percentages of the CD45RA- subsets, and several other fine subset alterations. The fine subset profile suggests, according to current correlations of phenotype and function, that both CD4+ suppressor inducer and CD4-dependent CD8+ suppressor effector cells are reduced, whereas other subsets, including CD8+ CTL or their precursors, are increased in relative percentages. Some of the phenotypic changes are reversible over the several days following withdrawal. In other results, the percentage of CD8hi lymphocytes epxressing CD11b (beta-integrin) is shown to be reciprocal with the percentage expressing L-selectin both in normals and alcoholics. However, the regression function of CD11b vs. L-selectin on CD8hi cells is different for the alcoholics than for the normals, indicating an abnormality in the regulation of the expression of these two adhesion markers. Taken together, this abnormality of adhesion molecules and the fine subset alterations previously described indicate widespread changes in the peripheral lymphocytes of currently drinking alcoholics. These changes suggest functional deficiencies that may include alterations of lymphocyte traffic and other adhesion-dependent functions, and a shift in the balance of regulatory interactions.

Expression of adhesion molecules on infiltrating T cells in thyroid glands from patients with Graves' disease

The present study was performed to elucidate the role of adhesion molecules in the pathogenesis of Graves' disease. Peripheral blood and intrathyroidal mononuclear cells were obtained from 14 patients with Graves' disease. The expression of adhesion molecules and HLA-DR antigen on CD4+ cells and CD4+ cell subpopulations was analysed by the two- or three-colour immunofluorescence method. The expression of adhesion molecules including LFA-1 alpha, LFA-1 beta, CD2, VLA-4 alpha and VLA-5 alpha on CD4+ cells in the thyroid gland was markedly higher than that in peripheral blood. In peripheral blood CD4+ cell subsets, the CD4+ CD45RO+ cell population had an enhanced expression of the adhesion molecules compared with the CD4+ CD45RA+ cell population. However, there was no significant difference in the expression of adhesion molecules by CD4+ cell populations and subsets between Graves' disease and healthy subjects. The thyroid gland from Graves' disease contained a higher percentage of CD4+ CD45RO+ cells and a lower percentage of CD4+ CD45RA+ cells. In intrathyroidal CD4+ cell subsets, the CD4+ CD45RO+ cell population had an increased expression of LFA-1 and CD2 compared with the CD4+ CD45RA+ cell population, but there was no significant difference in VLA-4 and VLA-5 expression between the two cell subsets. Furthermore, the expression of LFA-1 and CD2 on the CD4+ CD45RO+ cell population in the thyroid was significantly higher than that in matched peripheral blood. A similar finding was also observed for the CD4+ CD45RA+ cell population. The thyroid gland had an increased percentage of CD4+ HLA-DR+ cells compared with matched or healthy peripheral blood. However, there was no significant difference in the percentage of HLA-DR+ cells in the thyroid gland between CD4+ CD45RO+ cell and CD4+ CD45RA+ cell populations. These results suggest that increased expression of adhesion molecules on CD4+ cells may be responsible for the migration of these cells into thyroid glands and cellular interactions between these cells and thyroid epithelial cells.

Relationships between interleukin-6 activity, acute phase proteins, and function of the hypothalamic-pituitary-adrenal axis in severe depression

Recent studies from this laboratory have provided some evidence that major depression, in particular melancholia, may be accompanied by an immune response. The present study was designed to investigate whether severe depression is characterized by increased interleukin-6 (Il-6) activity and whether Il-6 production is related to altered levels of acute phase reactants and to abnormal function of the hypothalamic-pituitary-adrenal (HPA) axis. Measurements were made in 8 healthy control subjects and 24 depressed inpatients of Il-6 production in culture supernatants of mitogen-stimulated peripheral leukocytes and plasma levels of haptoglobin (Hp), transferrin (Tf), and postdexamethasone cortisol. Il-6 activity was significantly higher in melancholic subjects than in healthy control subjects and in patients with minor depression or nonmelancholic major depression. Il-6 production was significantly correlated with Hp (positively) and Tf (negatively) plasma levels. There were significant and positive correlations between Il-6 activity and postdexamethasone cortisol values. The findings may suggest that increased Il-6 activity in severe depression is related to hypotransferrinemia, hyperhaptoglobinemia, and hyperactivity of the HPA axis.

Immunohistological analysis of memory T lymphocytes and activated B lymphocytes in tissues with periodontal disease

Memory T-cells and activated B-cells were identified in cryostat sections of adult periodontitis (AP) lesions and categorized in terms of frequency and distribution. Nineteen periodontitis biopsies were obtained at the time of periodontal surgery to remove residual periodontal pockets following the completion of initial preparation. Gingival tissues exhibited various degree of inflammation (GI of 0-2) but probing depths of > 4 mm and > 5 mm loss of attachment. As a control, 5 gingivitis specimens (GI of 1, probing depth and loss of attachment of < or = 3 mm) were obtained from premolar and third molar sites requiring extraction for either orthodontic treatment or pericoronitis. Serial cryostat sections (6 microns in thickness) were prepared from each biopsy, on which a double staining avidin-biotin immunoperoxidase and avidin-biotin alkaline phosphatase technique was used to identify CD4+, CD45RO+ memory T-cells and activated CD19+ B-cells expressing CD23 or CD25. In periodontitis lesions, the mean percentage of CD4+ cells expressing CD45RO was consistently high (65.9% in the crevicular (C) one-third (1/3), 61.2% in the middle (M) 1/3 and 62.5% in the oral (O) 1/3). This contrasts with the low mean percentage of CD4+, CD45RA+ naive T-cells (17.1% in the C 1/3, 14.8% in the M 1/3 and 12.4% in the O 1/3). In gingivitis specimens, the incidence of CD4+, CD45RO+ was 81.9% in the C 1/3, 81.1% in the M 1/3 and 89.0% in the O 1/3. This was higher than that of periodontitis biopsies. With CD4+, CD45RA+ the incidence was 10.0% in the C 1/3, 8.0% in the M 1/3, and 6.6% in the O 1/3 and the relationship to the periodontitis biopsies was reversed. However, the percentage of CD23+ and CD25+, CD19+ B-cells which were identified in 13 out of 19 samples from periodontitis varied significantly (0-100% for CD23, 0-36.2% for CD25) in spite of similar clinical status. The frequency of B-cells and activated B-cells in the gingivitis was much lower than that of periodontitis. These results indicate that both T-cells and B-cells were in active stage in periodontitis lesions. Differences of immunohistological features between gingivitis and periodontitis may be attributable to the heterogeneity of profiles of cytokine production by CD4+, CD45RO+ "memory' cells.

Presence of T-cell subset abnormalities in newly diagnosed cases of multiple sclerosis and relationship with short-term clinical activity

Abnormalities of T-cell subsets in patients with multiple sclerosis are well known; in order to assess whether immunological abnormalities are relevant in the pathogenesis of the disease after its clinical onset, peripheral blood lymphocyte subsets (CD3+, CD4+, CD4(+)-CD45RA+, CD4+CD45RA-, CD8+, CD8+CD57+, CD57+, CD25+) were analysed serially in 25 patients at the first clinical episode suggestive of inflammatory demyelinating disease and in an equal number of age- and sex-matched controls. During the follow-up period (12-18 months, mean 14) 6 of 25 patients presented new relapses: in this subgroup of patients, significant changes in CD4+ ratio (% CD4+CD45RA-/%CD4+CD45RA+) were detected in comparison both with healthy controls and with clinically stable patients. Patients clinically stable at follow-up did not display immunological abnormalities, regardless of the presence or absence of cerebrospinal fluid and/or magnetic resonance imaging alterations consistent with multiple sclerosis. These findings suggest a possible prognostic role of early T-cell subset imbalance in multiple sclerosis.

Activation of T cells through a T cell-specific epitope of CD45

The 180- and 190-kDa isoforms of CD45 are preferentially expressed on the helper inducer (memory) subset of CD4 cells. In order to generate monoclonal antibodies against the extracellular domains of these isoforms and determine whether they could regulate the function and activation of these cells, we developed a mAb, anti-4H2D, by immunizing Balb/c mice with an isogenic mouse pre-B cell line expressing the human 190-kDa CD45 isoform. Anti-4H2D reacts with approximately 60% of T cells, 70% of CD4 cells, and 60% of CD8 cells. The CD4 cell population defined by this mAb corresponds functionally and phenotypically to that defined by the CD45RO+CD29+ subset. Western blotting demonstrated that anti-4H2D reacts primarily with the 190-kDa isoform of CD45 and to a minor extent, the 205- and 180-kDa CD45 isoforms. Interestingly, this mAb reacted with only a subpopulation of mature thymocytes and peripheral T cells, despite the fact that the 190-kDa CD45 isoform, as well as CD45RO and CD29, is more widely distributed on cells of hematopoietic origin. The 4H2D epitope was neuraminidase sensitive, indicating that anti-4H2D reacts with a carbohydrate epitope which is present on only a subset of the T cells containing the 190-kDa CD45 isoform epitopes. Functional studies showed that soluble anti-4H2D augmented T cell proliferation induced by the CD2 and CD3 pathways, and treatment of T cells with this mAb up-regulated [Ca2+]i flux induced by both anti-CD2 and anti-CD3 mAbs. These results suggest that the 190-kDa CD45 isoform on human CD4 cells is heterogeneous and that the 190-kDa isoform recognized by anti-4H2D regulates the function and activation of CD4 helper T cells.

CD7- T cells in rheumatoid arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is characterized by decreased expression of CD7 in the peripheral blood and in the synovium. The present study was designed to identify the basis for and functional consequences of this decreased expression.

METHODS

Peripheral blood lymphocytes from normal controls and from patients with RA or systemic lupus erythematosus (SLE), and T cell lines derived from rheumatoid synovium, were evaluated using 3-color fluorescence-activated cell sorter analysis.

RESULTS

Normal subjects and most SLE patients expressed homogeneous, bright CD7 on CD4+, CD45RA+ cells, whereas RA patients demonstrated a significantly increased proportion of CD7- cells. T cell lines derived from rheumatoid synovium demonstrated a striking deficiency of CD7 on CD4+, CD45RA- cells. CD4+, CD45RA+ cells from RA patients changed phenotype after in vitro activation to CD45RA negativity, with up-regulation of CD7. CD7-, CD4+, CD45RA- cells were assessed for their ability to induce pokeweed mitogen-driven IgM and IgM-rheumatoid factor synthesis, and they were found to be potent helper/inducer cells. An increased population of CD7-, CD4+ cells in peripheral blood was found to predict a low response to recall antigens.

CONCLUSION

The low expression of CD7 in RA may explain some of the immune abnormalities which may contribute to the pathogenesis of this disease.

CD45 isoform expression on human neonatal T cells: expression and turnover of CD45 isoforms on neonatal versus adult T cells after activation

Neonatal T cells are phenotypically similar to "naive" T cells from adult donors in the CD45 isoform expression. Despite the phenotypic similarity, large differences were found between neonatal and adult T cells when T cells were activated. After activation with PHA, adult CD45RA+ T cells began to express CD45RO and no loss of CD45RA expression had yet occurred at Day 3 post-stimulation. Three days after activation, CD45RA+ neonatal T cells also coexpressed CD45RO; however, in contrast to adult T cells, a marked loss of CD45RA was observed. We analyzed the rapid loss of CD45RA found in neonatal T cells. The de novo synthesis of CD45 isoforms in neonatal T cells was essentially the same as that in the adult T cells. Turnover of the CD45RA was very rapid in both resting adult and neonatal T cells. After activation with PHA, the turnover of CD45RA on adult T cells was decreased significantly, while the turnover of CD45RA on neonatal T cells was not changed after activation. Therefore, the regulation of CD45 isoform expression not only involves switches in alternative splicing, but also involves different regulation of turnover of these isoforms from the cell membrane.

Preferential interaction of the CD4+29+/45RA-subset of human CD4+ T lymphocytes with an antibody against the cell-membrane ganglioside GD3

This study shows that the two major subpopulations of CD4+ T lymphocytes defined on the basis of differential expression of the CD29 and CD45RA antigens, show significant differences in reactivity with a monoclonal antibody against the GD3 ganglioside. Double staining studies showed that the GD3 ganglioside is predominantly expressed on cells from the CD4+29+/45RA subsets. The preferential interaction of the CD4+29+/45RA cell subset with the anti-GD3 MoAb was further confirmed by the proliferative and calcium-flux studies. Accordingly, the reciprocal, CD4+(29-)/45RA+ subset was unable to proliferate in response to the anti-GD3 MoAb alone. Although it did show a significant mobilization of calcium ions and proliferation to IL-2 when stimulated with the anti-GD3 MoAb, these response were much less pronounced than the responses of the CD4+29+/45RA- subset. Finally, when two T-cell stimulating monokines, IL-1 and IL-6, were tested for the ability to modulate the anti-GD3 mediated proliferation, only the former, but not the latter was able to enhance the proliferation. Although the natural ligand for the GD3 ganglioside remains unknown, the data presented here provide further evidence in support of the notion that the T-cell surface molecules different from the T-cell receptor MHC-antigen complex may contribute to the preferential activation of one of the CD4+ T lymphocyte subsets.

CD45RA and CD45RO positive CD4 cells in human peripheral blood and periodontal disease tissue before and after stimulation with periodontopathic bacteria

Flow cytometric analysis was used to examine naive and primed or memory CD4 cells extracted from periodontal lesions compared with cells from peripheral blood of healthy subjects before and after stimulation with the periodontopathic bacteria, Porphyromonas gingivalis and Fusobacterium nucleatum. In peripheral blood, approximately 60% and 40% of CD4 cells were CD45RO+ and CD45RA+ respectively at day 0. Phytohaemagglutinin (PHA) induced CD45RO expression on almost 100% of CD4 cells. However, P. gingivalis and F. nucleatum stimulation did not cause any significant change in percentage of CD45RO+ CD4 cells except for a loss of antigen at day 6 together with re-expression at day 7, which also occurred on cells cultured in medium only. CD45RA expression on PHA and bacterial-stimulated peripheral blood CD4 cells remained fairly stable for the 10-d culture period. Greater than 90% CD4 cells extracted from healthy or marginal gingivitis (H/MG) and adult periodontitis (AP) lesions were CD45RO+ and this was maintained on AP cells throughout the 6-d culture period, except for a small decrease in the percentage of positive cells induced by P. gingivalis at day 3. Approximately 9% CD4 cells from H/MG tissue were CD45RA+, but about 22% AP cells expressed this antigen, and this increased again in P. gingivalis- and F. nucleatum-stimulated cultures after 3 d. Therefore, in peripheral blood P. gingivalis and F. nucleatum do not act as nonspecific T-cell mitogens and, in AP cells, these bacteria induce changes in phenotype, supporting previous data that although they may be polyclonal B-cell activators, they activate antigen specific T-cells.

IL-2 infusion abrogates humoral immune responses in humans

Although IL-2 infusion enhances cell-mediated cytotoxicity in patients with neoplastic disease, administration is paradoxically associated with a modest fall in total serum IgG and an increased risk of infection. We now show that the adverse effects of IL-2 infusion on the humoral immune system are substantial. Although IL-2 induces the B cell growth and differentiating factors IL-4 and IL-6, infusion abrogates primary antibody responses entirely and reduces secondary antibody responses 50-fold following antigen challenge. There is no evidence of the generation of cells with suppressive activity on B cells but IL-2 increases the ratio of circulating virgin:memory cells. These results may help to explain the increased rate of bacterial infection in patients receiving IL-2. As IL-2 plays a central role in the generation of an immune response, the finding that it is also sufficiently immunosuppressive to inhibit primary- and secondary-type antibody responses suggests that exploration of the underlying mechanisms may provide insights into immune system homeostasis and may offer new approaches to therapeutic immunosuppression.

Reversal of CD45R isoform switching in CD8+ T cells

Both CD4+ and CD8+ T cells express either CD45RA or CD45R0 isoform of CD45R in an exclusive way. Recent reports have shown that CD45RA+ T cells lose CD45RA and gain CD45R0 upon activation. This switching has been suggested to be irreversible although more recently, examples of reversal of CD45R isotype switching in CD4+ T cells have been reported. We report here that freshly isolated unprimed CD8+ T cells, when activated with PHA, temporarily lose CD45RA but reexpress an intermediate level of CD45RA 2-3 weeks after activation with PHA. This reversal seems to take place much more slowly in unprimed CD4+ T cells: the majority of CD4+ T cells that had lost CD45RA and gained CD45R0 remained CD45RA-CD45R0+ in 3 weeks after the stimulation. Also, long-term CD8+ CD45RA+ T cell lines stimulated with PHA or OKT3 showed even more rapid recovery of CD45RA while PPD-specific CD4+ T cell clones retained the original CD45R0 phenotype 3 weeks after stimulation with PPD or PHA.

Expression of CD45 isoforms by Epstein-Barr virus-transformed human B lymphocytes

CD45 is the most common protein tyrosine phosphatase (PTPase) in the membrane of white blood cells, serving as a potent regulator of lymphocyte activation and signal transduction. While the amino acid sequence of the intracellular domain of the molecule is conserved, that of the extracellular domain occurs in multiple isoforms, each of the result of alternative mRNA splicing. In T lymphocytes, the lowest relative molecular mass (Mr) form, CD45RO, is associated with acquisition of memory function, whereas the highest Mr isoform, CD45RA, occurs in "naive" T cells. Recently, B cells were also found to express CD45RO following in vitro activation. In order to more fully characterize the expression of CD45 on activated B cells, we have studied its appearance on Epstein-Barr virus-transformed (EBV-t) cells and have found heterogeneous expression of CD45RO and CD45RA. CD45RO expression was unstable with eventual loss by some EBV-t lines, and loss followed by reappearance in others. CD45RA and CD45RO varied independently whereas CD45 remained stable and high, suggesting a fluctuation in other CD45 isoforms. Immunostaining for CD45RB indicates that a probable 190-kDa isoform may be responsible for this observation. A similar bidirectional reversible shifting between CD45RA and CD45RO on T-cell lines has also been reported by Rothstein et al. In contrast to some reports on normal B cells, neither CD45RA nor CD45RO expression was associated with PCA-1 expression. Further evidence that these EBV-t lines may not correspond to a well-defined stage of B-cell differentiation is provided by the observation that a disproportionate loss of CD20 compared to CD19 was noted for several lines. The basis for the CD45 isoform switching, or any functional difference(s) in the expressed isoforms, is not yet known for human B cells.

Isoforms of the CD45 common leukocyte antigen family: markers for human T-cell differentiation

The diverse host defense and immunoregulatory functions of human T cells are performed by phenotypically heterogeneous subpopulations. Among the membrane antigens that are differentially expressed by reciprocal human T-cell subsets are the CD45RA and CD45RO isoforms of the common leukocyte antigen family, which have been hypothesized to identify "naive" and "memory" T cells, respectively. The CD45RA antigen is first expressed by T-lineage cells relatively late during their intrathymic maturation and continues to be expressed by most T cells in the immunologically naive neonate. With increasing age and antigenic exposure, however, CD45RA-/RO+ cells become more prevalent in the circulation and comprise the majority of cells in tissues. Analyses of the functional capabilities of CD4+CD45RA+ and CD4+CD45RO+ cells have shown that proliferative responses to "memory" recall antigens or the ability to provide help for antibody production are functions uniquely performed by CD4+CD45RA-/RO+ cells. The major immunoregulatory functions described for CD4+CD45RA+ cells involve suppression of immune responses, either directly or via the induction of suppressor activity by CD8+ cells. Two general models of differentiation have been proposed to describe the lineal relationship of these T-cell subsets. Although these subsets could represent mature, phenotypically and functionally stable progeny arising from separate differentiation pathways, there is considerable experimental support for the hypothesis that CD45RA-/RO+ cells are "memory" cells that derive from "naive" or "virgin" CD45RA+/RO- precursors via an activation-dependent postthymic differentiation pathway. Altered frequencies of CD45RA+ and CD45RO+ T cells have been observed in a variety of different clinical conditions, particularly diseases manifesting altered immune function. These findings have contributed new information concerning the physiological events regulating the in vivo generation of these T-cell subsets. In addition, they may provide clues to the pathogenetic processes associated with certain diseases.

The coexpression of CD45RA and CD45RO isoforms on T cells during the S/G2/M stages of cell cycle

The tyrosine phosphatase CD45 is alternatively spliced to generate isoforms of different molecular weights (180-220 kDa) which are differentially expressed on hematopoietic cells. Monoclonal antibodies reacting with either the 180-kDa (UCHL-1, CD45RO) or the 200- to 220-kDa (2H4, CD45RA) isoform have been used to subdivide T cell populations based on their expression of one or the other of these two epitopes. CD45RA T cells have "naive" characteristics of unresponsiveness to recall antigens and prominence in cord blood, while CD45RO T cells are considered "memory" T cells because they proliferate to recall antigens and increase following PHA activation of cord blood. However, we have recently demonstrated the expression of the CD45RA isoform on a subpopulation of CD45RO+ T cell clones, suggesting that CD45RA is not a universal marker for naive T cells. Using propidium iodide staining of the DNA to determine cell cycle stage, we now show that CD45RA expression is significantly higher on T cell clones during the S, G2, and M stages of cell cycle when compared to CD45RA expression on cells in Go and G1. Furthermore, CD45RA expression on cells undergoing mitosis is not limited to long-term activated T cell clones, as uncultured peripheral blood T cells in the S/G2/M phase express significantly more CD45RA. The percentage of T cells coexpressing CD45RA and CD45RO also increases following PHA activation, indicating that T cells in the process of division express both isoforms. These results suggest a potential role of the CD45RA isoform during the stages of cell cycle leading to mitosis.

CD45 isoforms associated with distinct functions of CD4 cells derived from unusual healthy donors lacking CD45RA- T lymphocytes

We now report two healthy individuals whose T lymphocytes were over 95% positive for CD45RA antigen expression. However, these donors normally expressed both the CD29 high (CD29+) and CD45RO high (CD45RO+) antigens on approximately 40 and 50% of their CD4 cells, respectively. Despite the strong CD45RA expression on the surface of almost all CD4 cells, the CD29 marker allowed T cells from these donors to be divided phenotypically into subsets having distinct in vitro function. CD4+CD29+ cells from these donors responded maximally to recall antigens such as TT and provided strong helper function for B cell Ig synthesis. In contrast, CD4+CD29- cells responded poorly to recall antigens and had poor helper function for B cell Ig synthesis, but had strong suppressor activity. Thus, CD29 antigen expression was still predictive of the in vitro functional activity as previously described for normal donors. Furthermore, biochemical analysis of the distribution of individual CD45 isoforms on the surface of these subsets of CD4 cells revealed distinct differences. The CD4+CD29 high (CD4+CD29+) subset of cells primarily expressed the 180-, 190-, and 205-kDa CD45 isoforms, while the CD4+CD29 low (CD4+CD29-) cells primarily expressed the 190-, 205-, and 220-kDa CD45 isoforms. These results suggest that despite the superficial phenotypic similarity of CD4 cells in these donors, distinctions in the distribution of both CD29 and the 180- and 220-kDa CD45 isoforms exist and might play a role in the different functions of freshly isolated CD4 lymphocytes.

Loss of activation-induced CD45RO with maintenance of CD45RA expression during prolonged culture of T cells and NK cells

The results of the present study show that activation-induced changes in CD45RA and CD45RO expression on T cells and natural killer (NK) cells are not unidirectional for all cells during a 5-week culture period. T cells and NK cells were generated from a resting subpopulation of peripheral blood mononuclear cells (PBMC) defined by sedimentation at Percoll high buoyant densities (p greater than 1.0640 g/ml) and unresponsiveness to IL-2. T cells were activated by a combination of PHA, sheep erythrocytes and IL-2-conditioned medium (IL-2-CM), and NK cells were activated by co-culture with gamma-irradiated malignant melanoma (MM-170) cells and IL-2-CM. Both T-cell and NK-cell cultures were maintained by subculture in IL-2-CM. NK cells and the CD45R(Abright)RO(dim/neg) subpopulation of T cells gained CD45RO following activation and this was accompanied by a two-fold decrease in CD45RA expression. In different cultures, CD45RO expression was not stable on 28-80% of T cells and 10-55% of NK cells. Cells with decreased CD45RO expression showed increased expression of CD45RA. Instability of CD45RO expression on cultured T cells and NK cells occurred at a time following the period of rapid cell growth when the cells were entering a quiescent phase. Both the CD4+ and CD8+ T-cell subpopulation showed similar changes in CD45 isoform expression. In contrast to the results obtained with the CD45R(Abright)RO(dim/neg) resting T cells, the CD45RO(bright)RA(dim/neg) subpopulation of resting T cells when activated and cultured under identical conditions retained CD45RO expression and remained CD45RAdim/neg. Thus a significant proportion of resting CD45R(Abright)RO(dim/neg) T cells is not related in a differentiation sequence to resting CD45RObrightRAdim/neg T cells, and therefore resting CD45RAbrightROdim/neg T cells and resting NK cells may be heterogeneous with respect to their activation history.

In the mouse the maturation stage of the peripheral CD4+ CD45RA+ subset is different from that of the CD8+ CD45RA+ subset

In vitro studies have suggested that the presence of CD45RA on subsets of CD4 and CD8 cells defines naive T cells and that, in response to antigen, CD45RA+ cells become CD45RA- along a differentiation pathway. To test the hypothesis that CD45RA+ cells are naive cells which have just left the thymus, young mice were thymectomized. This would be predicted to lead to a fall in the size of the peripheral pool of CD45RA+ T cells. However, the changes in the size of this pool would also be dependent on the life-span and self renewal capacity of the CD45RA+ T cells in the periphery. Therefore, to test the contribution of the thymus to the peripheral CD45RA+ pool, the percentage of CD45RA+ cells among spleen lymphocyte subsets was studied from 10 days up to 2 years of age in thymectomized and control mice. We also studied the expression of the memory marker CD44 on the CD45RA subsets of CD4 and CD8 cells, as well as the effect of in vitro activation on expression of CD45RA. Our results show that CD8+ CD45RA+ cells are mainly CD44- and their maintenance is dependent on the presence of the thymus. In contrast, the majority of CD4+ CD45RA+ are CD44+ and are not affected by thymectomy. This indicates that the maturation stage of CD8+ CD45RA+ cells is different from that of CD4+ CD45RA+ cells.

Differential responses of CD4+CD45RA+ and CD4+CD29+ subsets to activated CD8+ cells: enhanced stimulation of the CD4+CD45RA+ subset by cells from patients with multiple sclerosis

To examine whether functionally different CD4+ cells respond uniformly to the immunoregulatory influences of allogeneic activated CD8+ cells (*CD8+), we subfractionated the CD4+ population into two subsets, based on the high expression of either CD45RA or CD29. We confirmed that the CD45RA+ cells proliferated poorly in response to soluble anti-CD3 mAb, compared to the vigorous response obtained with the CD29+ subset; the CD45RA+ cells were more responsive to stimulation with Con A. Using normal healthy controls, we found that whereas *CD8+ had a significant suppressive effect on the proliferation of the CD29+ subset, they augmented the mitogen-induced proliferative response of the CD45RA+ cells. We further demonstrated that *CD8+ derived from MS patients augmented the response of the CD45RA+ subset to a significantly higher degree compared to healthy age- and sex-matched controls. There were no significant differences between the degree of suppression exerted by the *CD8+ of either the MS or the control group on the CD29+ cells. These results demonstrate that helper/memory CD4+CD29+ cells are more sensitive to the suppressive influences of *CD8+ compared to the CD4+CD45RA+ subset. In addition, in MS, *CD8+ may contribute to a more pronounced "on" signal for virgin CD4+CD45RA+ cells, which might serve as a means to perpetuate the autoimmune disease process.

Interleukin-1 corrects the defective autologous mixed lymphocyte response in multiple sclerosis

Patients with chronic progressive multiple sclerosis (MS) have alterations of T cell regulation that can be measured by in vitro assays and include decreases of the autologous mixed lymphocyte reaction (AMLR). Whether a defect in cytokine secretion was involved in the altered AMLR was investigated in 29 MS patients and 13 age- and sex-matched controls. The response of CD4+ T cell populations to irradiated non-T cells was decreased in MS as compared to control subjects. As previously reported, decreases in the AMLR were similarly observed with whole T cells of MS subjects as compared to controls. The addition of recombinant interleukin (IL)-1 to cultures of either whole T cells or CD4+ T cells with irradiated non-T cells in the AMLR corrected the immune defect in subjects with MS but had no effect on the AMLR in control subjects. In contrast, addition of rIL-2 or rIL-4 to the AMLR did not correct the decreased AMLR in MS patients as compared to controls. The lymphokines IFN-gamma and TGF-beta 2 both decreased the AMLR in MS patients and controls while TNF had no effect. Further, the magnitude of the AMLR response corresponded to IL-1 secretion induced by LPS in the non-T cell population. These studies indicate that defects in IL-1 may be related to immune defects of suppression in MS patients. Selective correction of immunoregulatory defects using lymphokines or their inducers in subjects with autoimmune diseases such as MS may be possible.

CD4+CD45RA+ and CD4+CD45RA- T cell subsets in man maintain distinct function and CD45RA expression persists on a subpopulation of CD45RA+ cells after activation with Con A

We have previously shown that Con A-induced suppressor T cells belong to the CD45RA+ subset. After unseparated T cells are activated with Con A, CD45RA expression increases to a maximum (Day 2), and then decreases significantly, but does not disappear entirely (Day 9), while CD29 expression increases steadily. In the present study, we examined the fate of these cell surface molecules on isolated CD4+CD45RA+ and CD4+CD45RA- cells following activation with Con A, and their relationship to the regulatory functions of these subsets. After activation of CD4+CD45RA+ cells with Con A, CD45RO and CD29 antigen expression rapidly increases (greater than 90%). While CD45RA expression is downregulated, approximately 40% of the cells continue to express low-density CD45RA in a stable fashion through Day 21. Despite these phenotypic changes, cells originally CD45RA+ continue to suppress IgG synthesis and provide only minimal B cell help. Furthermore, when cells originally CD45RA+ were sorted on the basis of continued presence, or loss of CD45RA antigen 14 days after activation, both populations demonstrated potent suppression and minimal help. In contrast, after activation with Con A, CD4+CD45A- cells maintain stable phenotype and provide significant help and minimal suppression. Immunoprecipitation of the CD45RA antigen from Day 14 activated CD4+CD45RA+ cells confirms the continued presence of the 205-kDa isoform, but reveals a significant decrease in the 220-kDa isoform. These results suggest that after activation with Con A, cells originally CD45RA+ remain functionally distinct from cells originally CD45RA-, and that CD45RA antigen persists on a subpopulation of CD45RA+ cells after activation with Con A.

Selective expression of CD45 isoforms on functional subpopulations of CD34+ hemopoietic cells from human bone marrow

We have found that the small population of cells in human marrow that are characterized by their expression of CD34 can be readily subdivided into two apparently nonoverlapping subpopulations of approximate equal size, one expressing CD45RO and one CD45R. Functional studies of these subpopulations revealed that all of the primitive erythroid colony-forming cells (BFU-E) are CD34+ CD45RO+. Similarly, more primitive cells that give rise to both erythroid and granulopoietic colony-forming cells after being maintained for 5 wk on confluent irradiated long-term marrow culture feeder layers, also show this phenotype. In contrast, most granulopoietic colony-forming cells are CD34+ CD45RO- cells. The differential expression of CD45 isoforms on distinct functional subpopulations of hemopoietic cells is consistent with the concept that these molecules play an important role in the differentiation or activation of primitive, normally quiescent, hemopoietic cells. The presence of CD45RO and the lack of CD45R on human cells capable of initiating hemopoiesis in the long-term marrow culture system correspond to the reported lack of CD45R on transplantable hemopoietic stem cells in rodents and may be a useful addition to strategies for human stem cell purification, or for purging CD45R+ leukemic cells.