CD5 positive B cells in patients with rheumatoid arthritis: phorbol ester mediated enhancement of detection

Hiding in plain sight: time to unlock autoimmune clues in human CD5+ B cells by using nextgen technology

CD5+ B cells expand in many autoimmune diseases, including type 1 diabetes (T1D), rheumatoid arthritis (RA), and systemic lupus erythematosus (SLE). Furthermore, CD5+ B cells contain important subsets: IL-10-producing B-reg cells, FasL-expressing subset, and the majority of pre-naive B cells. In addition, they are major sources of natural autoantibodies, which are polyreactive and autoreactive. Thus, CD5+ B cells are clearly loaded with autoimmune clues that are yet to be unlocked and understood. We hypothesize that human CD5+ B cells are likely to yield enormously important novel information about the role of B cells in autoimmune disease if analyzed using the new technological advances in molecular biology and genomics. Use of high-throughput sequencing of B cell receptors (BCR) of CD5+ B cells could reveal public BCRs associated with autoimmune diseases, whereas transcriptional analysis of CD5+ B cells using single-cell RNA-seq may delineate distinct sublineages and their relationship to conventional B cells. If it turns out that autoimmune repertoires are concentrated in CD5+ B cells, given that CD5+ B cells are clearly identifiable by flow cytometry, therapeutic strategies can be developed to safely remove CD5+ B cells to mitigate ongoing autoimmunity and protect at-risk individuals.

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

Bone resorption correlates with the frequency of CD5⁺ B cells in the blood of patients with rheumatoid arthritis

OBJECTIVE

The prevention of bone resorption and subsequent joint destruction is one of the main challenges in the treatment of patients suffering from RA. Various mechanisms have previously been described that contribute to bone resorption in tightly defined cohorts. Here we analysed a cross-sectional cohort of RA patients and searched for humoral and cellular markers in the peripheral blood associated with bone resorption.

METHODS

We enrolled 61 consecutive RA patients positive for ACPA. Blood was analysed by flow cytometry to determine the percentages of regulatory T cells and B cell subpopulations. Cytokine (TNF-α, IL-6, IL-10) and ACPA levels as well as the bone resorption marker CTX-1 were determined from the patients' sera. Standard clinical disease parameters were included.

RESULTS

Multivariate analyses showed that the percentages of CD5(+) B cells were positively correlated with CTX-1 serum levels. However, neither low-avidity ACPA nor serum IL-6 levels, both known to be produced by CD5(+) cells, were associated with CTX-1 in patients' sera. There was no correlation between CTX-1 levels and clinical parameters or ACPA levels.

CONCLUSION

In summary, we found that the CD5(+) B cell population is associated with bone resorption as measured via serum CTX-1 levels in a cross-sectional cohort of RA patients. However, a possible functional link between CD5(+) B cells and bone resorption still needs to be defined.

B Lymphocytes Are Required for Development and Treatment of Autoimmune Diseases

Recent studies have revealed that B cells serve extraordinarily diverse functions within the immune system in addition to antibody production. These functions contribute to autoimmunity. They initiate the development of lymphoid architecture and regulate dendritic and T-cell function through cytokine production. Receptor editing is also essential to prevent autoimmunity. Both abnormalities in the distribution of B-cell subsets and the benefits of ablative B-cell therapy of autoimmune states confirm their importance. Results from transgenic models have demonstrated that the sensitivity of B cells to antigen receptor cross-linking correlates to autoimmunity, with particular reference to negative signaling by CD5 and CD22. These mechanisms maintain tolerance by recruiting src-homology 2 domain-containing protein tyrosine phosphatase-1. These findings open new prospects for immunotherapy of autoimmune diseases.

Expression of RAGs in Peripheral B Cells outside Germinal Centers Is Associated with the Expression of CD5

Previous studies have indicated that mature B cells reactivate secondary V(D)J recombination inside and outside the germinal center (GC) of peripheral lymphoid organs. The nature of the B cells undergoing Ig rearrangement before they enter GC is unknown. In this study, we present evidence that activated mature CD5-positive human tonsil B cells coexpress both RAG1 and RAG2 mRNA and protein, and display DNA cleavage resulting from their recombinase activity. Furthermore, in vitro activation of CD5-negative naive mature B cells by IgR and CD40 cross-linking induces expression of CD5 on a subset of cells, and leads to the up-regulation of RAG1 and RAG2 only in cells turned positive for CD5. Thus, RAG gene expression is closely related to CD5 expression outside GCs. These data suggest that CD5 is associated with receptor revision in activated mature B cells and likely to promote expression of suitable IgR capable of initiating the GC reaction.

Phenotypic pattern of B cells in the appendix: reduced intensity of CD19 expression

The lymphoid tissue of the appendix is considered as part of the gut-associated lymphoid tissue (GALT). In order to understand better the immunological significance of the appendix we analyzed the cellular composition of normal and inflamed human appendix tissue by flow cytometer with special attention to expression of the CD19 and CD5 markers on B cells. Cellular analysis was also performed on peripheral and appendical vein blood samples as well as on omentum and peritoneal fluid samples. The study population included seventeen patients aged 2-15 yr. (mean age - 11.5 yr.) undergoing appendectomy. Ten children were diagnosed with acute appendicitis while 7 had a normal appendix.

RESULTS

Compared to the peripheral blood, the appendix contained a significantly higher percentage of CD19 cells (47.6% of total lymphocytes versus 15%, p<0.0001), and B1 cells (4.98% of total lymphocytes versus 2.42%, p=0.001). In addition, the intensity of CD19-staining was markedly decreased in the appendix (mean - 395.7), and also in the omentum (mean - 398.2) as compared to peripheral lymphocytes (mean - 497.7, p<0.0001 for both comparison). Comparison between the inflamed and the non-inflamed appendices revealed that the inflamed appendix contained a significantly higher proportion of B1 cells (5.64% of total lymphocytes versus 3.53%, p=0.032), and also a higher B1/b cell ratio (0.13 vs. 0.07, p=0.01).

CONCLUSIONS

These results indicate that the appendix tissue contains higher number of B1 (and B) cells compared to the peripheral blood and that these cells play a role in the primary immune response to acute infection/inflammation in the appendix. Appendiceal B cell population is unique in term of CD19 intensity expression on their surface.

CD5 B cells and B-cell malignancies

Over the past year, progress has been made in understanding of the physiology and disease associations of CD5+ (B1) B cells, although their exact role in pathogenesis remains unclear. Earlier studies on the negative function of CD5 within the B-cell receptor complex have been substantiated, and it seems likely that soon the signaling pathways used by this coreceptor will be elucidated. Progress in diagnosis, physiology, and etiopathogenesis of CD5+ malignancies has been made, particularly in B-cell chronic lymphocytic leukemia. The low-level expression of surface immunoglobulin has been explained by the mutations that occur in the associated CD79b. Two new potential tumor-suppressor genes have been identified in the hot spot of chromosome 13q, which provides an exciting step forward in understanding of the etiopathogenesis of some B-cell chronic lymphocytic leukemia. Activated signal transducers for activation of transcription factors molecules have been shown to be phosphorylated on different amino acids in B1 and chronic lymphocytic leukemia tumors, although the significance of this is, as yet, unclear. Finally, aberrant expression of CD40L by chronic lymphocytic leukemia T cells may contribute to the immunodeficiency that develops in these patients.

CD5 B cells in autoimmune and non immune-mediated thyroid dysfunctions

In a previous study we demonstrated a significant increase of CD5+ B subset in patients with Graves' disease (GD) compared with normal controls. The aim of this study was to compare the percentage of CD5+ B and CD5- B cells in GD with that in different forms of autoimmune and non immune-mediated thyroid diseases. Seventy-two patients were studied: 28 patients with GD, 20 with silent thyroiditis (ST), 12 with Hashimoto's disease (HD), and 12 subjects affected by hyperthyroidism due to toxic adenoma (TA). Eleven out of 28 patients with GD were also evaluated after six months of methimazole treatment. The study was performed by cytometric analysis. In GD the percentage and the absolute number of CD5+ B cells were significantly increased compared with normal controls (42.5 +/- 18.2% versus 19 +/- 6.3%, p < 0.0001; 142 +/- 153.3/cmm versus 46.9 +/- 22/cmm, p < 0.003, respectively. CD5+ B cells tended to normalise after six months of treatment. In ST the percentage of CD5+ B cells was increased (28.6 +/- 10.2%); conversely the absolute number was in the normal range. Patients affected by HD did not show any significant modification in B cells and their subsets in comparison with controls. In TA, CD5+ B were 7.6 /- 4.4% and 14.3 /- 10.9/cmm. Our results demonstrated a marked increase in both percentage and absolute number of CD5+ cells, only in active GD. The expansion of CD5+ B cells could play a role in the immune imbalance present in this disease.

B lymphocytes and humoral immune responses in rheumatoid arthritis

Although the etiology of rheumatoid arthritis (RA) remains unknown at present, major evidence implicates humoral immune mechanisms in the pathophysiology of this disease. The primary lesions occur in the synovium and are characterized by immunological activation and chronic inflammation. Activated B cells and plasma cells secrete antibodies including rheumatoid factors and anticollagen antibodies. These antibodies give rise to immune complex formation and activation of the inflammatory cascade. There is no evidence that rheumatoid factor secreting B cells act as antigen presenting cells and present immune complex to T cells, leading to perpetuation of the inflammatory response.

CD5 expression on B cells may be an activation marker for secretion of anti-myelin antibodies in patients with polyneuropathy associated with monoclonal gammopathy

B cells expressing the CD5 marker belong to a subpopulation with potential autoreactive properties. Increased proportions of CD5+ B cells have been reported in autoimmune diseases. In patients with monoclonal gammopathy and demyelinating polyneuropathy, the M-component often consists of autoantibodies reacting with myelin components. We therefore investigated if CD5+ B cells were involved in the production of anti-myelin antibodies. There was no difference of mean value of CD5+ B cells between patients and controls. However, the proportion of CD5+ B cells was significantly correlated with the amount of anti-myelin antibodies. In seven patients, CD5+ B cells were enriched using an immunomagnetic technique. The number of CD5+ and CD5- B cells secreting anti-myelin antibodies was determined by ELISPOT. In two patients with high levels of antibodies, antibody-secreting cells were mainly, but not exclusively, CD5+ B cells. In five patients with low levels of antibodies, most cells secreting anti-myelin antibodies were CD5-. We conclude that CD5 expressed on B cells may be an activation marker, reflecting B cells producing high amounts of anti-myelin antibodies in patients with polyneuropathy associated with monoclonal gammopathy.

CD5+ B cells and the immune system

The CD5+ B-cell population is prominent in early life and may play a key role in the ontogeny of the immune system. Transplantation studies in mice are in support of CD5+ B cells as a separate lineage from CD5- B cells. In both mice and men there is evidence in favour of CD5 being an activation antigen rather than a lineage marker, but the jury is still out! The frequency of CD5+ B cells appears to be under genetic influence. CD5+ B cells are receptive to many cytokines including IL-2 and IL-5 and themselves produce a number of cytokines especially IL-10. The function of the CD5 molecule on B cells is presently unknown but it might be involved in interaction with CD72 on other B cells. CD5+ B cells generally utilise minimally mutated germ-line genes and produce low avidity auto- and polyreactive antibodies (natural antibodies) generally of the IgM class.

Human CD5-positive B cells in lymphoid malignancy and connective tissue diseases

The current literature on human CD5-positive B cells (CD5 + B cells) has been analysed, with a special emphasis on non organ-specific auto-immune diseases. Malignant cells of most of the chronic lymphoid leukaemias of the B cell lineage express the CD5 molecule. Antibodies of the IgM class produced by leukaemic B cells are multispecific auto-antibodies. The CD5 + B cell subset may be expanded in non organ-specific autoimmune diseases, such as rheumatoid arthritis, primary Sjögren's syndrome, systemic lupus erythematosus. This holds true for various conditions, including organ-specific auto-immune diseases. Since auto-immune features are common in lymphoproliferative disorders, and the latter be a complication in non organ-specific auto-immune diseases, CD5 + B cells may represent an intermediary between these auto-immune diseases and B cell lymphoproliferations. Studies on the regulation of CD5 + B cell production and function are likely to shed light on the aetiology of, and pathogenetic mechanisms operating in the different disease states.

CD5 and immunoglobulin VH gene expression in B-cell lines from patients with autoimmune diseases

We studied the CD5 mRNA expression and VH gene family usage in Epstein-Barr virus (EBV)-immortalized B-cell lines derived from the blood of patients with type 1 diabetes (IDDM) of recent onset and of patients with polyneuritis cranialis multiplex (cranial neuritis; CN). After immortalization with EBV, at least 10 cell lines from each subject were tested for surface CD5 and CD20. mRNA expression was studied using cDNA probes for the six VH families as well as for CD5. The EBV lines from the IDDM patients used the VHIV family more frequently and VHI and VHII families less frequently than lines from controls. EBV lines from CN patients expressed the VHI and VHII families more often than those of the controls. When the IDDM and CN lines were compared, the lines derived from IDDM patients were found to use VH families I and II less frequently and VH families IV and V more frequently than lines from CN patients. There were no significant differences in the mean numbers of CD5+ B cells in the cell lines tested. More than half of the lines from each patient expressed CD5 at the mRNA level. No correlation was seen between the expression of surface CD5 and the level of CD5 mRNA expression. There was, however, a positive correlation between the usage of VH families III, V and VI, and the CD5 mRNA expression. In conclusion, the usage of VH families I to VI seemed to differ in patients with IDDM and CN. No differences were seen in the surface CD5 expression, but the lines expressing CD5 mRNA preferentially used the VH families III, V and VI.

Analysis of the expression of CD5 by human B cells and correlation with functional activity

B cells expressing the CD5 marker in the mouse have been suggested to be a separate lineage and a major source of autoantibody production. In man, this relationship is less clear. Studies were therefore undertaken to determine whether human CD5+ B cells represent a distinct lineage of cells that differ in patterns of antibody production from CD5- B cells. In normal B cell populations, CD5 was expressed by a mean of 24.0 +/- 2.8% (n = 10) of CD20+ B cells. Of note, an increased frequency of CD5+ B cells was not found in patients with systemic lupus erythematosus (mean of 17.9 +/- 2.8%, n = 16). Analyzing CD5+ B cells for cell membrane Ig isotype expression demonstrated similar frequencies of IgG and IgA expressing cells as were found on the CD5- B cell population, although the frequency of IgM+ cells was slightly increased. Incubation of CD20+ B cells with phorbol myristate acetate (PMA) for 72 hr increased the frequency of CD5 expressing B cells by more than threefold. CD5 expression was also increased by coculture with anti-CD3-activated T cells and most markedly by simultaneous stimulation with both PMA- and anti-CD3-activated T cells (greater than 50% positive). Analysis of CD5- B cells clearly indicated that stimulation with PMA or anti-CD3-activated T cells induced the majority to become CD5+ transiently. Functional analysis of Ig production by CD5+ and CD5- B cells stimulated with anti-CD3-activated T cells indicated that both populations in normals produced IgM and a variety of autoantibodies in comparable amounts, whereas the CD5- B cells produced greater quantities of IgG. B cells were activated with anti-CD3-stimulated T cells followed by separation into CD5+ and CD5- populations. The largest amount of Ig was produced by CD5- B cells that were induced to express CD5, although all populations produced some Ig. These data suggest that CD5 behaves as an activation marker on human B cells rather than as a marker for a distinct lineage of cells. Moreover, CD5 expression does not appear to identify a population of resting B cells with a greater capacity to produce antibodies to DNA or other autoantibodies.

Effect of gold therapy on CD5+ B-cells and TCR gamma delta+ T-cells in patients with rheumatoid arthritis

Circulating CD5+ B-cell levels in 15 patients with rheumatoid arthritis (RA) not receiving remittive therapy was significantly increased when compared to 17 normal controls (mean +/- SE: RA, 19.7 +/- 2.85%; controls, 11.6 +/- 1.67%; P less than 0.02). In contrast, 24 patients with RA receiving gold sodium thiomalate therapy (GST) had similar CD5+ B-cell levels (11.88 +/- 1.65) when compared to controls and significantly reduced levels when compared to the RA group not receiving remittive agents (P less than 0.01). Furthermore, TCR gamma delta+ T-cell levels were also assessed in these patients groups. These values were not significantly different between any of the groups (controls, 4.46 +/- 1.36%; GST, 6.88 +/- 1.73%; RA, 2.73 +/- 0.55%), although 42% of the GST treated group had gamma delta+ T-cell levels higher than the entire untreated RA group. No correlation was observed between the levels of TCR gamma delta + T-cells and CD5+ B-cells in any of these groups. These results suggested that therapy does influence the level of CD5+ B-cells and gamma delta+ T-cells in these patients.

CD5+ B cells and CD4-8-T cells in neuroimmunological diseases

Using 2- and 3-colour FACS analysis we found increased levels of fetal-type CD5+ B cells and CD4-8- T cells in cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS) and aseptic meningitis (AM) compared to control probands with muscular tension headache (TH). Similar differences were found for CD5+ B cells in peripheral blood, but at lower levels. CD4-8- T cells in blood exceeded those in CSF in all patient groups, with the exception of relapsing remitting MS, revealing the highest values in AM. There was a positive correlation between CD4-8- T cells and T cell receptor (TCR) gamma delta bearing T cells in blood and CSF. The double-negative T cells exceeded the TCR gamma delta T cells by about 1%. A positive correlation between CD5+ B cells and CD4-8- T cell level in CSF was found in MS and AM, but not in TH, nor in blood of any patient group. HLA-DR expression was lower in CD5+ B cells than in CD5- B cells. We conclude that fetal-type lymphocytes are enriched in CSF compartment of patients with inflammatory diseases of the central nervous system, irrespective of autoimmune mechanisms involved, but the function of CD5+ B cells is mainly to produce the autoantibodies.

Auto- and polyreactivity of IgM from CD5+ and CD5- cord blood B cells

The presence of the CD5 (67 kDa) molecule on the surface of B cells has been considered a marker for cells producing auto- and polyreactive antibodies. Cord blood B lymphocytes (rich in CD5+ B cells) have been sorted into CD5 positive and negative populations by flow cytometry using monoclonal antibodies to CD20 and CD5. Clones of these populations were obtained by immortalization with Epstein-Barr virus. Clones derived from both CD5+ and CD5- B cells produced IgM which was auto- and polyreactive with a higher frequency of these specificities in the CD5+ population. These data indicate that expression of surface CD5 on cord blood B cells is not a definitive marker of an auto/polyreactive population.

Influence of host environment on growth of clonal CD5+B (Lyl+B) cells

Autoimmune NZB mice have increased percentages of CD5+B (Lyl+B) cells in both the spleen and peritoneum. We have previously reported that as NZB mice age they develop a clonal population of hyperdiploid CD5+B cells in the spleen. These cells can readily be transplanted into unirradiated recipients. The growth characteristics of such transplanted hyperdiploid NZB spleen cells were examined in different recipient strains to determine if the immunological status of the host environments affected the growth of the clonal CD5+B cells. Young NZB and NZB.xid recipients (lacking hyperdiploid CD5+B cells) allowed growth and expansion of unpassaged CD5+B cells derived from primary NZB mice. Similarly, (NZBxDBA/2) and (NZBxBALB/c) F1 recipients allowed for expansion of CD5+B cell clones from primary sources. In a separate experiment, T cell-depleted NZB spleen cells containing a hyperdiploid CD5+B cell clone were transferred to SCID mice. The SCID environment supported the growth of the primary clone. None of these recipients normally have elevated CD5+B cells, yet these recipients allowed growth of primary transferred hyperdiploid cells. However, a difference in the ability of these recipient strains in their ability to expand multiply passaged CD5+B cell clones was observed. These results indicate that while hyperdiploid CD5+B cells are difficult to be maintained in culture, they can readily be passaged in vivo. The host environment may provide growth factors or signals for endogenous growth factors. Although the CD5+B clones arise initially in a hyperactive autoimmune environment, a hyperimmune environment is not necessary to support their growth. Transferred CD5+B cells affect the recipient environment and reduce the percentages of normal B cells.

Possible immunoregulatory role for CD5 + B cells

CD5 + B cells represent a subpopulation of B cells which have the characteristic of employing unmutated immunoglobulin variable region genes. These cells are found to be increased early in ontogeny. The percentage of CD5 + B cells is highest in the fetus and decreases after birth. The antibodies produced by CD5 + B cells are polyreactive and are the natural autoantibodies. These autoantibodies may not be pathogenic. CD5 + B cells are elevated in certain autoimmune disease states and are the malignant cell type in B-CLL, with a strong genetic component involved in determining elevated CD5 + B cell states. Elevated CD5 + B cells are found in immunodeficient states (young, aged, and autoimmune). CD5 + B cells may normally act as a first-line defense against invading foreign pathogens but are not involved in the specific immune response. There is some evidence, at least in newborns, that CD5 + B cells may affect the emerging B cell repertoire of conventional B cells via idiotype cascade. However, the action of CD5 + B cells in the newborn may be quite different than their activity in the adult. Nonimmunoglobulin-producing CD5 + B cells may be immunosuppressors. In this report, a unique subpopulation of CD5 + B cells was investigated. These cells were found only in the spleens of aged NZB mice. The CD5 + B cells were clonal and possessed extra chromosomes and did not appear to be producing antibodies. These cells were capable of rapid proliferation in unirradiated recipients. By taking advantage of this proliferative capability, the effect of exogenous clonal CD5 + B cells on recipient immune system was evaluated. Clonal CD5 + B cells from NZB mice were immunosuppressive and decreased the numbers of conventional B cells as well as the level of "natural antibodies." In summary, CD5 + B cells may play different roles in the immune system depending upon environment, age, and their differentiation state (i.e., proliferation versus antibody secretion). The natural antibody produced by CD5 + B cells may be involved in maintenance functions such as removal of dead cells and first-line defense mechanisms. In addition, CD5 + B cells may themselves regulate the immune system and produce a factor which is immunosuppressive. An understanding of the various functions of CD5 + B cells may elucidate fundamental immunoregulatory circuits.

The relationship between CD5-expressing B lymphocytes and serologic abnormalities in rheumatoid arthritis patients and their relatives

The influence of genetic factors on the expression of CD5+ B lymphocytes and their relationship to a broad spectrum of autoantibodies was investigated in a study of 12 patients with rheumatoid arthritis (RA) and 52 of their healthy first-degree relatives. The proportion of CD5+ B cells was significantly higher in RA patients (mean +/- SEM 23.9 +/- 2.7%) compared with that in their relatives (18.3 +/- 1.1%, P less than 0.05) and compared with that in a group of healthy control subjects (16.1 +/- 1.8%; P less than 0.05). Much more striking, however, were the high levels of CD5+ B cells found in the patients and their relatives in 5 of the families studied. Increases in total immunoglobulin levels and autoantibody levels were frequently observed in RA patients (approximately 20-40%) and their relatives (approximately 10-20%). Furthermore, a statistically significant correlation (P less than 0.01) between IgM rheumatoid factor and the percentage of B lymphocytes expressing CD5 was observed.

CD5 mRNA expression and auto-antibody production in early human B cells immortalized by EBV

A number of studies have suggested that lymphocytes producing polyreactive antibodies belong to the CD5+ B-cell subset. In this study we have examined CD5 at the cell surface and mRNA levels in EBV-driven cord blood and fetal liver clones previously characterized in terms of their antibody specificities. We show that EBV-immortalized cells can express surface CD5, and that some of the clones not expressing surface CD5 express it at the mRNA level. The complete absence of CD5 mRNA in some polyreactive clones is consistent with the proposition that the production of auto-antibodies and multispecific antibodies is not restricted to the CD5+ B-cell subset.

The antibody repertoire of early human B cells. I. High frequency of autoreactivity and polyreactivity

Cord blood and fetal liver B cells were immortalized using Epstein-Barr virus, and IgM antibodies from the resulting lines and clones were examined for their binding to a variety of auto-antigens and micro-organisms by ELISA and fluorescence assays. Auto-antigens tested included Fc of IgG, ssDNA and dsDNA, cardiolipin, histones 1-4, collagens type I and II, thyroglobulin, cytoskeletal components, and a tissue section screen. Of 71 cell lines tested, all but 19 showed some autoreactivity. All 32 fetal liver lines reacted to some self-antigens. In cord blood clones, 16 out of 26 bound to auto-antigens. Many of the clones reacted with more than one auto-antigen and were 'polyreactive'. Some of the cord blood clones bound to extracts of micro-organisms, showing specificity for both endogenous and exogenous antigens. The high frequency of CD5+ B cells in the cord blood (greater than 50%) and fetal liver (greater than 70%) argues for many of these clones being derived from this subset. Therefore, our data support the concept that many 'early' B cells produce polyreactive IgM which can bind to a variety of different auto-antigens and micro-organisms. These IgM antibodies are similar to those described by others as 'natural antibodies'.

Cell surface antigen CD5 is a marker for activated human B cells

A minor subset of B cells which in vivo express the surface antigen CD5, has attracted much attention because of its involvement in autoimmune responses. On the basis of observations showing self-renewal capacity of such cells in mice and also the absence of a substantial change of CD5 phenotype during B cell activation in vitro, the CD5+ B cells are now generally considered to represent a separate cell lineage. In the present study, CD5- B cells were isolated by cell sorter and then stimulated in vitro with mutagenized EL4 thymoma cells in the presence of T cell supernatant. About 70% of the B cells were CD5+ after 3 days. Thus, the CD5 antigen behaves as a B cell activation marker. In our system we found that the frequency of rheumatoid factor-producing B cells was on average three times higher in CD5+ than in CD5- B cells isolated ex vivo from human peripheral blood. Most likely this reflects frequent activation of such autoreactive B cells in vivo.

Immunopathogenesis of Sjogren's syndrome: "facts and fancy"

Sjogren's syndrome (Ss) is an ideal model to study the pathogenesis of both autoimmunity and malignancy. It occurs as an organ specific autoimmune disease, alone or in association with almost every other autoimmune disorder, as a systemic disorder, and finally it can evolve to B-cell-lymphoid malignancy. The most consistent finding in the syndrome, the B-cell-hyperreactivity, follows the same steps of evolution. It starts as polyclonal, but not random, since the autoantibody profile correlates with the disease subgroups and the systemic manifestations and it seems to be controlled by the MHC gene composition. Further, in the systemic form of the disease it presents as a poly-oligo-mono-clonal process and ends up to monoclonal (IgMk) B-lymphoid malignancy. Studies on the T-immunoregulatory subsets and function can not explain this B-cell hyperreactivity. The initial trigger is unknown. Estrogens, known as immunoenhancers possibly promote the B-cell hyperreactivity and certain genes controlling HLA class-II MHC molecules may represent susceptibility factors for the development of the disease. The discovery of lymphokines and particularly the B-cell growth and differentiation factors as well as the rapid development of the retro-virology field may give answers pertinent to the pathogenesis of Ss and to B-cell lymphoid malignancy.

Stimulatory effect of CD5 antibody on B cells from patients with rheumatoid arthritis

In order to clarify the role of CD5 antigen on B cell in autoimmunity, we examined B cells from patients with rheumatoid arthritis (RA). The percentages of CD5 positive B cells were increased in peripheral blood from RA compared with normal. Normal and RA B cells were stimulated with two kinds of monoclonal antibodies to CD5 (Leu-1, SL-1) which recognize different epitopes. RA B cells proliferated and secreted IgM by CD5 antibody stimulation in combination with IL-1. Our observations imply that CD5 positive B cells in RA are in their differentiation stage and that CD5 antigen might be one of the triggers to activate CD5 positive B cells in vivo to produce autoantibody.

Chronic lymphocytic leukemic (CLL) cells secrete multispecific autoantibodies

A subset of B cells expressing the CD5 marker, a 67 KD molecule, has been implicated in the pathogenesis of autoimmune disease. To study the immunoglobulin repertoire of CD5+ B cells we investigated chronic lymphocytic leukemic (CLL) cells, since the majority of the malignant clones express CD5. CLL were induced to secrete their IgM in vitro by phorbol 12-myristate 13-acetate (PMA) and the supernatants screened for binding to a panel of autoantigens. Twelve out of 14 CLL clones were autoreactive, binding to Fc of IgG, ssDNA, dsDNA, histones, cardiolipin, or cytoskeletal components. Many also bound to more than one antigen tested for, showing multispecificity. Our data suggest that a high proportion of CD5+ B cells are programmed to secrete multispecific autoantibodies.

CD5-expressing B lymphocytes in the blood and salivary glands of patients with primary Sjögren's syndrome

CD5, the human counterpart of Ly-1 molecules in the mouse, are detectable but weakly expressed on a minute fraction of circulating B cells. The number of CD5 + B cells in the blood of patients with Sjögren's syndrome was slightly higher than in control blood, but it became statistically significant after treatment of the cells with phorbol myristic acetate. These numbers were even higher in patients with homogeneous serum bands than in the others. A few scattered cells were stained with anti-human IgM antibody on salivary gland sections, and among them 5-10% were found to be positive for anti-CD5.

The importance of CD5-positive B cells in nonorgan-specific autoimmune diseases

CD5, the human counterpart of Ly-1 molecules in the mouse, are detectable but weakly expressed on a minute fraction of circulating B cells. The number of CD5-expressing B cells is increased in patients with rheumatoid arthritis or primary Sjögren's syndrome. These cells are similar to those leading to chronic lymphocytic leukemia (they may be induced to produce multispecific autoantibodies). Multispecific autoantibodies have also been described in the early B cell repertoire.