CD2 is involved in maintenance and reversal of human alloantigen-specific clonal anergy

Revisiting T-cell adhesion molecules as potential targets for cancer immunotherapy: CD226 and CD2

Cancer immunotherapy aims to initiate or amplify immune responses that eliminate cancer cells and create immune memory to prevent relapse. Immune checkpoint inhibitors (ICIs), which target coinhibitory receptors on immune effector cells, such as CTLA-4 and PD-(L)1, have made significant strides in cancer treatment. However, they still face challenges in achieving widespread and durable responses. The effectiveness of anticancer immunity, which is determined by the interplay of coinhibitory and costimulatory signals in tumor-infiltrating immune cells, highlights the potential of costimulatory receptors as key targets for immunotherapy. This review explores our current understanding of the functions of CD2 and CD226, placing a special emphasis on their potential as novel agonist targets for cancer immunotherapy. CD2 and CD226, which are present mainly on T and NK cells, serve important functions in cell adhesion and recognition. These molecules are now recognized for their costimulatory benefits, particularly in the context of overcoming T-cell exhaustion and boosting antitumor responses. The importance of CD226, especially in anti-TIGIT therapy, along with the CD2‒CD58 axis in overcoming resistance to ICI or chimeric antigen receptor (CAR) T-cell therapies provides valuable insights into advancing beyond the current barriers of cancer immunotherapy, underscoring their promise as targets for novel agonist therapy.

Detection of Aberrant CD58 Expression in a Wide Spectrum of Lymphoma Subtypes: Implications for Treatment Resistance

CD58 or lymphocyte function-associated antigen-3, is a ligand for CD2 receptors on T and NK cells and is required for their activation and target cell killing. We recently showed a trend toward higher frequency of CD58 aberrations in patients with diffuse large B-cell lymphoma (DLBCL) who progressed on chimeric antigen receptor-T-cell treatment compared with those who responded. Given that CD58 status may be an important measure of T-cell-mediated therapy failure, we developed a CD58 immunohistochemical assay and evaluated CD58 status in 748 lymphomas. Our results show that CD58 protein expression is downregulated in a significant proportion of all subtypes of B-, T-, and NK-cell lymphomas. CD58 loss is significantly related to poor prognostic indicators in DLBCL and to ALK and DUSP22 rearrangements in anaplastic large-cell lymphoma. However, it is not associated with overall or progression-free survival in any of the lymphoma subtypes. As eligibility for chimeric antigen receptor-T-cell therapy is being extended to a broader spectrum of lymphomas, mechanisms of resistance, such as target downregulation and CD58 loss, may limit therapeutic success. CD58 status is therefore an important biomarker in lymphoma patients who may benefit from next-generation T-cell-mediated therapies or other novel approaches that mitigate immune escape.

CD58 Immunobiology at a Glance

The glycoprotein CD58, also known as lymphocyte-function antigen 3 (LFA-3), is a costimulatory receptor distributed on a broad range of human tissue cells. Its natural ligand CD2 is primarily expressed on the surface of T/NK cells. The CD2-CD58 interaction is an important component of the immunological synapse (IS) that induces activation and proliferation of T/NK cells and triggers a series of intracellular signaling in T/NK cells and target cells, respectively, in addition to promoting cell adhesion and recognition. Furthermore, a soluble form of CD58 (sCD58) is also present in cellular supernatant in vitro and in local tissues in vivo. The sCD58 is involved in T/NK cell-mediated immune responses as an immunosuppressive factor by affecting CD2-CD58 interaction. Altered accumulation of sCD58 may lead to immunosuppression of T/NK cells in the tumor microenvironment, allowing sCD58 as a novel immunotherapeutic target. Recently, the crucial roles of costimulatory molecule CD58 in immunomodulation seem to be reattracting the interests of investigators. In particular, the CD2-CD58 interaction is involved in the regulation of antiviral responses, inflammatory responses in autoimmune diseases, immune rejection of transplantation, and immune evasion of tumor cells. In this review, we provide a comprehensive summary of CD58 immunobiology.

CD2 Immunobiology

The glycoprotein CD2 is a costimulatory receptor expressed mainly on T and NK cells that binds to LFA3, a cell surface protein expressed on e.g., antigen-presenting cells. CD2 has an important role in the formation and organization of the immunological synapse that is formed between T cells and antigen-presenting cells upon cell-cell conjugation and associated intracellular signaling. CD2 expression is upregulated on memory T cells as well as activated T cells and plays an important role in activation of memory T cells despite the coexistence of several other costimulatory pathways. Anti-CD2 monoclonal antibodies have been shown to induce immune modulatory effects in vitro and clinical studies have proven the safety and efficacy of CD2-targeting biologics. Investigators have highlighted that the lack of attention to the CD2/LFA3 costimulatory pathway is a missed opportunity. Overall, CD2 is an attractive target for monoclonal antibodies intended for treatment of pathologies characterized by undesired T cell activation and offers an avenue to more selectively target memory T cells while favoring immune regulation.

The nature of activatory and tolerogenic dendritic cell-derived signal II

Dendritic cells (DCs) are central in maintaining the intricate balance between immunity and tolerance by orchestrating adaptive immune responses. Being the most potent antigen presenting cells, DCs are capable of educating naïve T cells into a wide variety of effector cells ranging from immunogenic CD4⁺ T helper cells and cytotoxic CD8⁺ T cells to tolerogenic regulatory T cells. This education is based on three fundamental signals. Signal I, which is mediated by antigen/major histocompatibility complexes binding to antigen-specific T cell receptors, guarantees antigen specificity. The co-stimulatory signal II, mediated by B7 family molecules, is crucial for the expansion of the antigen-specific T cells. The final step is T cell polarization by signal III, which is conveyed by DC-derived cytokines and determines the effector functions of the emerging T cell. Although co-stimulation is widely recognized to result from the engagement of T cell-derived CD28 with DC-expressed B7 molecules (CD80/CD86), other co-stimulatory pathways have been identified. These pathways can be divided into two groups based on their impact on primed T cells. Whereas pathways delivering activatory signals to T cells are termed co-stimulatory pathways, pathways delivering tolerogenic signals to T cells are termed co-inhibitory pathways. In this review, we discuss how the nature of DC-derived signal II determines the quality of ensuing T cell responses and eventually promoting either immunity or tolerance. A thorough understanding of this process is instrumental in determining the underlying mechanism of disorders demonstrating distorted immunity/tolerance balance, and would help innovating new therapeutic approaches for such disorders.

Congenital B cell lymphocytosis explained by novel germline CARD11 mutations

Germline mutations in CARD11 that result in constitutive NF-κB activation and selective B cell expansion underlie congenital B cell lymphocytosis.

Nuclear factor-κB (NF-κB) controls genes involved in normal lymphocyte functions, but constitutive NF-κB activation is often associated with B cell malignancy. Using high-throughput whole transcriptome sequencing, we investigated a unique family with hereditary polyclonal B cell lymphocytosis. We found a novel germline heterozygous missense mutation (E127G) in affected patients in the gene encoding CARD11, a scaffolding protein required for antigen receptor (AgR)–induced NF-κB activation in both B and T lymphocytes. We subsequently identified a second germline mutation (G116S) in an unrelated, phenotypically similar patient, confirming mutations in CARD11 drive disease. Like somatic, gain-of-function CARD11 mutations described in B cell lymphoma, these germline CARD11 mutants spontaneously aggregate and drive constitutive NF-κB activation. However, these CARD11 mutants rendered patient T cells less responsive to AgR-induced activation. By reexamining this rare genetic disorder first reported four decades ago, our findings provide new insight into why activating CARD11 mutations may induce B cell expansion and preferentially predispose to B cell malignancy without dramatically perturbing T cell homeostasis.

Alefacept in corticosteroid refractory graft versus host disease: Early results indicate promising activity

Steroid refractory graft versus host disease (GVHD) presents a significant therapeutic challenge due to the limited efficacy and safety of second-line treatments. Three patients with extensively pretreated, refractory GVHD were treated with a targeted anti-T-cell agent, alefacept, and demonstrated rapid and clinically significant improvement in their GVHD, facilitating tapering of corticosteroids. The pathological and immunohistochemical findings of GVHD also improved, validating our clinical impression. These preliminary findings indicate that alefacept may have beneficial activity in GVHD warranting further study.

Human intestinal intraepithelial lymphocytes and epithelial cells coinduce interleukin-8 production through the CD2-CD58 interaction

Human intestinal CD3+TCRalphabeta+CD8+ intraepithelial lymphocytes (IELs) are intimately associated with epithelial cells (ECs) through binding of CD103 to E-cadherin. How these two cell types functionally interact is largely unknown. IEL-EC cross talk was determined using HT-29 cells as the model EC and IL-8 as the readout. IL-8 was derived from both cell types and synergistically increased when the cells were combined. This synergistic effect required active transcription by both IELs and HT-29 cells. Cell contact was required as shown by the loss of the synergistic increase in IL-8 when the two cell types were separated by Transwells. Specifically, IL-8 release required the binding of CD2 on the IELs to CD58 on the HT-29 cells. The association of the CD3/TCR complex with major histocompatibility antigen class I antigens was not involved. Antibody neutralization of tumor necrosis factor-alpha (TNF-alpha), but not interferon-gamma (IFN-gamma), resulted in increased IL-8 production by the coculture. Although both TNF-alpha and IFN-gamma increased IL-8 synthesis and CD58 expression by the HT-29 cells, only IFN-gamma reduced IL-8 production by IELs. IL-8 production by either cell type involved phosphorylation of p38 and JNK. In summary, the synergistic synthesis of IL-8 occurs when IELs are stimulated through the CD2 pathway by CD58 on HT-29 cells, resulting in TNF-alpha release that, in turn, augments IL-8 synthesis and CD58 expression by the HT-29 cells.

Protein interactions between CD2 and Lck are required for the lipid raft distribution of CD2

In T lymphocytes, lipid rafts are preferred sites for signal transduction initiation and amplification. Many cell membrane receptors, such as the TCR, coreceptors, and accessory molecules associate within these microdomains upon cell activation. However, it is still unclear in most cases whether these receptors interact with rafts through lipid-based amino acid modifications or whether raft insertion is driven by protein-protein interactions. In murine T cells, a significant fraction of CD2 associates with membrane lipid rafts. We have addressed the mechanisms that control the localization of rat CD2 at the plasma membrane, and its redistribution within lipid rafts induced upon activation. Following incubation of rat CD2-expressing cells with radioactive-labeled palmitic acid, or using CD2 mutants with Cys226 and Cys228 replaced by alanine residues, we found no evidence that rat CD2 was subjected to lipid modifications that could favor the translocation to lipid rafts, discarding palmitoylation as the principal mechanism for raft addressing. In contrast, using Jurkat cells expressing different CD2 and Lck mutants, we show that the association of CD2 with the rafts fully correlates with CD2 capacity to bind to Lck. As CD2 physically interacts with both Lck and Fyn, preferentially inside lipid rafts, and reflecting the increase of CD2 in lipid rafts following activation, CD2 can mediate the interaction between the two kinases and the consequent boost in kinase activity in lipid rafts.

CD4+ T cells from simian immunodeficiency virus disease-resistant sooty mangabeys produce more IL-2 than cells from disease-susceptible species: involvement of p300 and CREB at the proximal IL-2 promoter in IL-2 up-regulation

IL-2 is an important cytokine required for the physiological function of CD4(+) T cells. Immunological unresponsiveness-anergy- of CD4(+) T cells is characterized by the inability of these cells to synthesize IL-2. Both progressive HIV infection leading to AIDS in humans and SIV infection in rhesus macaques (RM) are associated with dysregulation of IL-2 synthesis. In certain nonhuman primate species, such as sooty mangabeys (SM), SIV infection does not lead to AIDS. We have shown that this is associated with the resistance of the CD4(+) T cells from SM to undergo anergy in vitro. In this study, we show that CD4(+) T cells from SM spontaneously synthesize 2- to 3-fold higher levels of IL-2 than corresponding cells from RM. Proximal IL-2 promoter constructs derived from SM show significantly higher activity than the RM-derived constructs in primary CD4(+) T cells, which is associated with an element at approximately nt -200. Activity of both constructs was up-regulated by p300 and down-regulated by CREB to a similar degree. Chromatin immunoprecipitation analysis showed significantly higher binding of p300 and lower binding of CREB to the SM promoter in vivo. Two single nucleotide substitutions present in the SM sequence around position -200 and -180 seem to increase the affinity of these sites for the binding of transcription factors, one of which was identified as Oct-1. These unique characteristics of the proximal IL-2 promoter in SM therefore can represent one of the mechanisms contributing to the resistance of these cells to undergo anergy.

Anti-CD2 Monoclonal Antibody and Tacrolimus in Adult Liver Transplantation

BACKGROUND

Blockade of costimulation and adhesion signaling is an attractive approach to interfere with graft rejection

METHODS

Between January 1997 and May 1999, forty adults having benign liver diseases were included in a prospective, randomized study comparing tacrolimus plus low-dose short-term steroids without (n=20, TAC group) or with a 10-day course of antihuman CD2 monoclonal antibody (n=20, BTI group).

RESULTS

At day 7, histological rejection expressed by mean Banff scores (2.3+/-1.6 vs. 5.4+/-1.6 in the TAC group; P<0.0001) and incidence of moderate to severe rejection (score>or=6) (0 vs. 10 [50%] in the TAC group; P<0.001) were significantly lower in the BTI group. Rejection was treated in 10% (two patients) of BTI patients during the first 3 months and in 15% during the whole follow-up and in 25% (five patients) of TAC patients (P=NS). None of the BTI-patients presented with an adverse event. Three-month, 1-year, and 5-year actual patient survival rates were 100%, 95%, and 95% in the BTI group and 100%, 100%, and 85% in the TAC group. Graft survival rates were 100%, 90%, and 90% in the BTI group and 95%, 95%, and 80% in the TAC group (P=NS). The mAb had no negative impact on infectious or tumor events.

CONCLUSIONS

Antihuman CD2 monoclonal antibody is a safe immunosuppressive drug which has a favorable impact on early immunological follow-up of liver transplanted patients. The antibody had no impact on late patient and graft survival.

Leishmania donovani: Effect of therapy on expression of CD2 antigen and secretion of macrophage migration inhibition factor by T-cells in patients with visceral leishmaniasis

Visceral leishmaniasis (VL) commonly known as Kala-azar in India is one of the several clinically important infections, where Th1 sub-population of CD4+ T-cells, despite a pre-requisite, fails to express macrophage migration inhibition factor (MIF) and interferon-gamma which both activate the macrophage and coordinate the immune response to intra-cellular Leishmania sp. Expression of CD2 receptors before and after antileishmanial therapy on CD4+ T-cells of VL patients and their corresponding effect on MIF were examined. Before treatment the number of T-cells expressing CD2 was low which incorporated insignificant MIF response. The immunological reconstitution was, however, observed after treatment as manifested through upregulation of CD2+ T-cells with pronounced MIF generation response. The study, therefore, identifies a possible role of CD2 antigen in immunity to VL.

T lymphocyte-endothelial cell interactions

Human vascular endothelial cells (EC) basally display class I and II MHC-peptide complexes on their surface and come in regular contact with circulating T cells. We propose that EC present microbial antigens to memory T cells as a mechanism of immune surveillance. Activated T cells, in turn, provide both soluble and contact-dependent signals to modulate normal EC functions, including formation and remodeling of blood vessels, regulation of blood flow, regulation of blood fluidity, maintenance of permselectivity, recruitment of inflammatory leukocytes, and antigen presentation leading to activation of T cells. T cell interactions with vascular EC are thus bidirectional and link the immune and circulatory systems.

Immunotherapeutic effects of T11TS/S-LFA3 against nitrosocompound mediated neural genotoxicity

Nitrosocompounds formed by the interaction of nitrites and secondary amines are neurotoxic in human and different rodent species. Human exposure of nitrosocompounds are widespread affected by different modes like nitrite/nitrate preserved foods, beverages like beer, formed in the stomach following uptake of the precursors nitrates, nitrites and secondary amines. The productions of alkylating metabolites during the breakdown of nitrosocompounds are the causative agents for the neurotoxic changes of the neural cells. An attempt has been made in our lab to study the effect of nitrosocompound mediated toxicity and the gradual toxic effects of these neurotoxic agents to transform the normal glial cells to a neoplastic one. The present study indicated that a transmembrane glycopeptide of sheep red blood cell (SRBC), known as S-LFA3 or T11 target structure (T11TS) applied to nitrosocompound induced animals manifesting a full grown intracranial malignancy can revert back tumor-bearing condition to the normal physiological state. Young Druckray rat of both sexes aging 3-5 days were injected with N'-N'-ethyl nitrosourea (ENU) intraperitoneally (i.p.) at a single dose of 80 mg/kg body weight to simulate nitrosocompound mediated neurotoxicity. 2-,4-, 6-, 8-, and 10-month-old neonatal ENU induced animals were sacrificed for growth kinetics, functional immunological parameters and receptor studies to hint at the changes during tumor development. In order to determine the immunomodulatory role of T11TS, 7-month-old ENU induced animals were injected with T11TS at a dose of 0.41 mg/kg body weight, in three consecutive doses at an interval of 6 days maintaining normal control as untreated control and ENU induced animals of age-matched rats as tumor-bearing control. All the immunological parameters, growth kinetic study, receptor-based study by FACS directly established the immunomodulatory, anti-toxic and anti-tumor property of T11TS/S-LFA3. Finally, formation of DNA ladder along with the FACS-based apoptosis study clearly indicated that T11TS is a potent apoptotic inducer in neoplastic neural cells.

CD2 engagement induces dendritic cell activation: implications for immune surveillance and T-cell activation

We have shown previously that primary dendritic cells and monocytes express equal levels of CD14 but are distinguishable by the presence of CD2 on dendritic cells. CD2 is known to mediate the activation of T and natural killer (NK) cells through its interaction with CD58. CD2 epitopes recognized by anti-T111, -T112, and -T113 monoclonal antibodies (mAbs) are present on dendritic cells. Here we show that CD2 engagement significantly increases class II, costimulatory (CD40, CD80, CD86), adhesion (CD54, CD58), and CCR7 molecule expression on primary dendritic cells. Conversely, minimal or no change in the expression of the above antigens occurs on monocyte-derived dendritic cells, because these molecules are already maximally expressed. However, both kinds of dendritic cells release interleukin-1beta (IL-1beta) and IL-12 after CD2 engagement. Lastly, interference with dendritic cell CD2-T-cell CD58 engagement decreases naive CD4+CD45RA+ T-cell proliferation. Collectively, our results suggest another role of the CD2-CD58 pathway that allows nonimmune and immune cells to interact directly with dendritic cells and initiate innate and adaptive immune responses.

T cell anergy

T cell anergy is a tolerance mechanism in which the lymphocyte is intrinsically functionally inactivated following an antigen encounter, but remains alive for an extended period of time in a hyporesponsive state. Models of T cell anergy affecting both CD4(+) and CD8(+) cells fall into two broad categories. One, clonal anergy, is principally a growth arrest state, whereas the other, adaptive tolerance or in vivo anergy, represents a more generalized inhibition of proliferation and effector functions. The former arises from incomplete T cell activation, is mostly observed in previously activated T cells, is maintained by a block in the Ras/MAP kinase pathway, can be reversed by IL-2 or anti-OX40 signaling, and usually does not result in the inhibition of effector functions. The latter is most often initiated in naïve T cells in vivo by stimulation in an environment deficient in costimulation or high in coinhibition. Adaptive tolerance can be induced in the thymus or in the periphery. The cells proliferate and differentiate to varying degrees and then downregulate both functions in the face of persistent antigen. The state involves an early block in tyrosine kinase activation, which predominantly inhibits calcium mobilization, and an independent mechanism that blocks signaling through the IL-2 receptor. Adaptive tolerance reverses in the absence of antigen. Aspects of both of the anergic states are found in regulatory T cells, possibly preventing them from dominating initial immune responses to foreign antigens and shutting down such responses prematurely.

The therapeutic potential of costimulatory blockade with CTLA4Ig in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic, systemic, inflammatory disorder characterised by joint inflammation and destruction. Controversy exists regarding the significance and exact role of activated T cells in RA. CTLA4Ig is a soluble fusion protein (cytotoxic T lymphocyte-associated antigen 4 fused to the heavy chain constant region of human IgG1), which represents the first in a new class of agents called costimulation blockers. By blocking the second signal required for optimal T-cell activation, CTLA4Ig has demonstrated efficacy in a variety of spontaneous and induced animal models of autoimmunity. A Phase II clinical study in RA showed CTLA4Ig was efficacious with an acceptable safety profile. These results suggest that activated T cells may be important in RA pathogenesis and that costimulation blockade may be a novel therapeutic approach for this disorder.

CD2 is a dominant target for allogeneic responses

CD2 and 2B4 (CD244) are members of the immunoglobulin gene superfamily and are both ligands for another family member, CD48. CD2 is widely distributed on T, NK, and B cells and some antigen-presenting cells, while 2B4 is expressed on NK and some T cells and monocytes and is known to participate in NK cytotoxicity. Since indefinite allograft survival could be obtained by a combination of anti-CD48 plus anti-CD2 mAb administration, it was important to determine the role of 2B4 blockade in allograft rejection. MAbs directed against CD2, CD48, or 2B4 were administered singly or in pairs to cardiac allograft recipients. The experiments show that only anti-CD2 plus anti-CD48 mAbs result in indefinite allograft survival, while anti-CD2 plus anti-2B4 mAbs substantially prolong graft survival, and anti-CD48 plus anti-2B4 mAbs were no better than each mAb alone. The effect of these mAbs on anti-CD3 mAb and alloantigen-driven proliferation and IFN-gamma production were also assessed. In general, anti-CD2 inhibited both anti-CD3 mAb and alloantigen-driven responses, while anti-CD48 inhibited only anti-CD3 mAb but not alloantigen-driven proliferative and cytokine responses. Anti-2B4 mAbs were generally ineffective alone. Combinations of mAbs were more effective than single mAbs only in alloantigen-driven proliferation, commensurate with allograft survival results. Using CD2-/- and CD48-/- T cells and antigen-presenting cells, we also demonstrate that these inhibitory mAbs act primarily by blocking intercellular interactions, rather than directly delivering negative signals to T cells. These results suggest that, unlike CD2, 2B4 is not a potent regulatory molecule or ligand for CD48 in the response to alloantigen. Blocking the 2B4-CD48 receptor-ligand pair does not inhibit T-cell responses and alloreactivity to the same degree as CD2-CD48 blockade.

A feline CD2 homologue interacts with human red blood cells

A cDNA encoding a feline homologue of CD2 (fCD2) was identified. Several amino acids (aa) important for ligand interaction, molecular folding or signal transduction, found in other mammalian CD2, were found to be highly conserved in the predicted fCD2 aa sequence. fCD2-expressing cells were able to form rosettes with human red blood cells (probably via human CD58), and the rosette formation was inhibited by an anti-fCD2 monoclonal antibody. These results are indicative of the similarity of feline and human CD2 structures. fCD2 was found to be expressed in feline peripheral blood T lymphocytes, monocytes and cultured lymphoid cells.

CD2 facilitates differentiation of CD4 Th cells without affecting Th1/Th2 polarization

The role of CD2 in murine CD4 helper T cell differentiation and polarization was examined using TCR-Cyt-5CC7-I transgenic recombination activating gene-2-/- H-2(a) mice on CD2+/+ or CD2-/- backgrounds. In the absence of CD2, thymic development was abnormal as judged by reduction in the steady state number of total, double-positive, and CD4 single-positive (SP) thymocytes, as well as a defect in their restorative dynamics after peptide-induced negative selection in vivo. In addition, in CD2-/- animals, lymph node CD4 SP T cells manifest a 10- to 100-fold attenuated activation response to cytochrome c (CytC) agonist peptides as judged by induction of CD25 and CD69 cell surface expression or [(3)H]TdR incorporation; differences in the magnitude of responsiveness and requisite molar peptide concentrations were even greater for altered peptide ligands. Although the presence or absence of CD2 did not impact the final Th1 or Th2 polarization outcome, CD2 expression reduced the CytC peptide concentration threshold necessary to facilitate both Th1 and Th2 differentiation. In vivo administration of CytC peptide to CD2-/- animals yielded an impaired CD4 SP T cell effector/memory phenotype compared with similarly treated CD2+/+ mice. Analysis of TCR-Cyt-5CC7-I human CD2 double-transgenic mice similarly failed to reveal a preferential Th1 vs Th2 polarization. Collectively, these results indicate that CD2 is important for the efficient development of CD4 SP thymocytes and TCR-dependent activation of mature CD4 lymph node T cells, but does not direct a particular helper T cell subset polarity.

T cell signal transduction and the role of CD7 in costimulation

The complex cellular interactions that govern the mammalian immune response are now known to include specific receptor/ligand interactions, recruitment of intracellular signaling molecules, activation of both kinases and phosphatases, and redistribution of macromolecular complexes into specific subcellular membrane locations that, in aggregate, result in transcriptional activation. While the TCR-CD3 signal is critical for activation of the resting T cell, it alone is not sufficient to initiate transcriptional activation or generate an effective immune response. A number of other coreceptor molecules, including CD4, CD8, and CD28, have now been characterized that also play important roles in initiating or amplifying the activation of the T cell. A 40 kDa member of the immunoglobulin superfamily, the CD7 molecule, has also been shown to have costimulatory activity and to induce tyrosine and lipid kinase activities. Here we will review the signaling pathways initiated by TCR, CD28, and CD7, as well as the functional consequences of signal transduction through these receptors.

Molecular dissection of the CD2-CD58 counter-receptor interface identifies CD2 Tyr86 and CD58 Lys34 residues as the functional "hot spot"

The heterophilic CD2-CD58 adhesion interface contains interdigitating residues that impart high specificity and rapid binding kinetics. To define the hot spot of this counter-receptor interaction, we characterized CD2 adhesion domain variants harboring a single mutation of the central Tyr86 or of each amino acid residue forming a salt link/hydrogen bond. Alanine mutations at D31, D32 and K34 on the C strand and K43 and R48 on the C' strand reduce affinity for CD58 by 47-127-fold as measured by isothermal titration calorimetry. The Y86A mutant reduces affinity by approximately 1000-fold, whereas Y86F is virtually without effect, underscoring the importance of the phenyl ring rather than the hydroxyl moiety. The CD2-CD58 crystal structure offers a detailed view of this key functional epitope: CD2 D31 and D32 orient the side-chain of CD58 K34 such that CD2 Y86 makes hydrophobic contact with the extended aliphatic component of CD58 K34 between CD2 Y86 and CD58 F46. The elucidation of this hot spot provides a new target for rational design of immunosuppressive compounds and suggests a general approach for other receptors.

Differentiation of regulatory T cells 1 is induced by CD2 costimulation

Induction and maintenance of peripheral tolerance is an important phenomenon for the control of homeostasis in the immune system. There is now compelling evidence for CD4(+) T cells that prevent immune pathology, both in autoimmunity and in transplantation. However, the mechanisms involved in the specific differentiation of these T cells are unknown. We had previously shown that repetitive stimulations of naive T cells in the presence of IL-10 induce the differentiation of T regulatory cells 1. We further dissected the mechanism of IL-10 function and demonstrated that IL-10 acts by the down-regulation of most costimulatory molecules without modifying the expression of CD58. Using artificial APCs expressing various costimulatory molecules, we demonstrated that, in contrast to other costimulation patterns, costimulation via CD2 alone, in the absence of costimulations through CD28- or LFA-1, induced T cell anergy in an IL-10-independent pathway along with the differentiation of Ag-specific regulatory T cells. T regulatory cell-1 differentiation via CD2 was very efficient as both high IL-10 secretion and regulatory function were observed after the first stimulation of naive T cells with CD32-CD58 L cells. The possibility to rapidly induce the differentiation of Ag-specific regulatory T cells will certainly accelerate their characterization and their potential use as regulators of T cell-mediated diseases.

Mechanisms of tolerance induction: blockade of co-stimulation

Induction of tolerance to transplantation antigens is believed to be a promising way to achieve long-term allograft survival without a deleterious immunosuppressive regimen. T-cell activation, which is an essential feature of graft rejection, requires a first signal provided by T-cell receptor (TCR) ligation and a second signal provided by engagement of co-stimulatory molecules with their respective ligands on antigen-presenting cells. The coordinated triggering of these two independent signalling systems ensures the full T-cell activation, including proliferation and acquisition of effector function. TCR occupancy in the absence of co-stimulatory signals leads to a sustained loss of antigen responsiveness called clonal anergy, which could be of major importance in transplantation. In vivo, co-stimulation blockade was indeed shown to allow for long-term allograft survival in several transplantation models. However, the current continuous identification of new co-stimulatory molecules suggests that a functional redundancy of the system exists and that tolerance to transplantation antigens might be achieved more easily through the combined blockade of two or several co-stimulatory signals. In this review, we analyse the biological effects of the disruption of some co-stimulation pathways in vitro and in vivo and discuss their potential interest for tolerance induction.

Enhancement of cALL immunogenicity by co-culture with a CD154 expressing 293 cell line

Pre-B cell acute lymphoblastic leukaemia (cALL) commonly occurs in young patients and although successful conventional therapies are available (such as cytotoxic drugs and bone marrow transplantation) for a proportion of patients (approximately 30%) these are ultimately unsuccessful. Recurrence of disease is a result of the failure of the immune system to recognize these abnormal cells and down-regulation of crucial molecules required for cognate CD4(+) T cell recognition has been postulated as a means of immune escape. In this study we show that an embryonic kidney cell line (293 cells) transfected with CD154 (40 L.1) are capable of not only maintaining the viability of primary ALL cells in culture but can also up-regulate the expression of a number of crucial molecules involved in antigen recognition. We show that 40 L.1 cell stimulation of primary ALL cell cultures can not only enhance the allogeneic and autologous MLR response to such cells but will also induce CTL effectors which are capable of lysing wild-type autologous ALL cells. It is therefore conceivable that such an approach could be used to generate an active anti-tumour response in patients, following conventional therapy, reducing the incidence of recurrence.

Dynamic Recruitment of Human CD2 into Lipid Rafts

CD2 mediates T cell adhesion via its ectodomain and signal transduction utilizing its 117-amino acid cytoplasmic tail. Here we show that a significant fraction of human CD2 molecules is inducibly recruited into lipid rafts upon CD2 cross-linking by a specific pair of mitogenic anti-CD2 monoclonal antibodies (anti-T11(2) + anti-T11(3)) or during cellular conjugate formation by CD58, the physiologic ligand expressed on antigen-presenting cells. Translocation to lipid microdomains is independent of the T cell receptor (TCR) and, unlike inducible TCR-raft association, requires no tyrosine phosphorylation. Structural integrity of rafts is necessary for CD2-stimulated elevation of intracellular free calcium and tyrosine phosphorylation of cellular substrates. Whereas murine CD2 contains two membrane-proximal intracellular cysteines, partitioning CD2 into cholesterol-rich lipid rafts constitutively, human CD2 has no cytoplasmic cysteines. Mapping studies using CD2 point mutation, deletion, and chimeric molecules suggest that conformational change in the CD2 ectodomain participates in inducible raft association and excludes the membrane-proximal N-linked glycans, the transmembrane segment, and the CD2 cytoplasmic region (residues 8-117) as necessary for translocation. Translocation of CD2 into lipid rafts may reorganize the membrane into an activation-ready state prior to TCR engagement by a peptide associated with a major histocompatibility complex molecule, accounting for synergistic T cell stimulation by CD2 and the TCR.

Human alloantigen-specific anergic cells induced by a combination of CTLA4-Ig and CsA maintain anti-leukemia and anti-viral cytotoxic responses

The success of allogeneic hematopoietic stem cell transplantation from HLA-disparate donors depends on the development of new strategies for graft-versus-host disease prevention able to target specifically donor antihost alloreactivity, while preserving GVL and antiviral immune surveillance. Recent experimental and clinical work has shown the feasibility of an approach based on induction of anergy to host alloantigens through blockade of B7/CD28 costimulatory signal in donor T cells, but data on the impact of this strategy on the recovery of the immune system are still lacking. We devised an ex vivo method for induction of host alloantigen-specific unresponsiveness based on treatment with the B7/CD28 blocking agent CTLA4-Ig associated with CsA. We then proceeded to assess the maintenance of an effective immune response towards viral pathogens and tumor cells after CTLA4-Ig/CsA treatment, by measuring the frequency of CTL precursors directed against CMV- and EBV-infected targets, and against autologous leukemic blasts. We demonstrated that (1) CTLA4-Ig and CsA can act synergistically in inducing a state of unresponsiveness to alloantigens; (2) the number of leukemia-reactive, EBV-specific and CMV-specific CTLp is not impaired by CTLA4-Ig/CsA treatment. Our data provide the first direct in vitro evidence that it is possible to preserve antiviral and antileukemia effector cells after blockade of CD28/B7 interaction during MLR.

Multiple co-stimulatory signals are required for triggering proliferation of T cells from human secondary lymphoid tissue

Vaccine-based therapies are being developed for a variety of cancers and their efficacy will be determined by their ability to stimulate T cells in the secondary lymphoid tissue. We found that T cells isolated from human secondary lymphoid organs (LT-T), in contrast to peripheral blood T cells (PB-T) are hyporesponsive to cross-linked anti-CD3 mAb (CD3c) even in the presence of exogenous IL-2. Using mAb to trigger CD2 and CD28 co-stimulatory molecules, we found that such dual co-stimulation of LT-T induces profound and sustained responses including CD25 expression, IL-2 secretion and proliferation. Different levels of co-stimulation produced a hierarchical pattern of responses in LT-T, which correlated with the degree of CD3-TCR down-regulation. Mature antigen-presenting cells (APC) restored the capacity of LT-T to proliferate to stimulation of the CD3-TCR complex. Blocking studies demonstrated that optimal proliferation was critically dependent on co-stimulation via CD2 and CD28 engaged by their ligands on the APC. Therefore, LT-T have increased co-stimulatory requirements as compared to PB-T, i.e. multiple co-stimulatory signals coupled to CD3-TCR triggering. Furthermore, LT-T were found to be dependent on APC for survival, in contrast to PB-T. Clearly, LT-T do not behave in a comparable way to PB-T and in vitro experiments assessing novel cancer vaccines should therefore use LT-T as the most appropriate population of responder T cells.

A critical role for CD2 in both thymic selection events and mature T cell function

To examine the function of CD2 in vivo, N15 TCR transgenic (tg) RAG-2(-/-) H-2(b) mice bearing a single TCR specific for the vesicular stomatitis virus octapeptide bound to the H-2K(b) molecule were compared on a wild-type or CD2(-/-) background. In N15tg RAG-2(-/-) CD2(-/-) mice, thymic dysfunction is evident by 6 wk with a pre-TCR block in the CD4(-)CD8(-) double-negative thymocytes at the CD25(+)CD44(-) stage. Moreover, mature N15tg RAG-2(-/-) CD2(-/-) T cells are approximately 100-fold less responsive to vesicular stomatitis virus octapeptide and unresponsive to weak peptide agonists, as judged by IFN-gamma production. Repertoire analysis shows substantial differences in Valpha usage between non-tg C57BL/6 (B6) and B6 CD2(-/-) mice. Collectively, these findings show that CD2 plays a role in pre-TCR function in double-negative thymocytes, TCR selection events during thymocyte development, and TCR-stimulated cytokine production in mature T cells.

The involvement of the proto-oncogene p120 c-Cbl and ZAP-70 in CD2-mediated T cell activation

The CD2 co-receptor expressed on the surface of T lymphocytes is able to stimulate T cell activation, proliferation and cytokine production in the absence of direct engagement of the antigen-specific TCR. Engagement of human CD2 by mitogenic pairs of anti-CD2 mAb induces tyrosine phosphorylation of a number of intracellular proteins including a 120 kDa phosphoprotein that we identify as the proto-oncogene c-Cbl. Rapidly tyrosine phosphorylated following stimulation of a number of cell surface receptors, c-Cbl is an adaptor protein that has been shown to associate with a complex of intracellular signaling molecules, and to mediate both positive and negative regulatory effects. Here we show that, like TCR-CD3 stimulation, stimulation of CD2 enhanced the association of c-Cbl with both Crk(L) and the p85 subunit of phosphatidylinositol-3 kinase. Overexpression of wild-type c-Cbl protein inhibited both CD2and CD3-induced NF-AT transcriptional activity, suggesting that CD2 signaling is also negatively regulated by c-Cbl. The inhibitory effect of c-Cbl depended upon its N-terminal phosphotyrosine-binding domain, the domain that has been shown to be required for inhibition of the Syk/ZAP-70 family kinases. In Syk(-) Jurkat T cells stably expressing wild-type ZAP-70, CD2 stimulation induced only a minimal increase in ZAP-70 tyrosine phosphorylation. Nevertheless, ZAP-70 kinase was required for CD2-mediated NF-AT transcriptional activity. Thus, CD2-mediated NF-AT transcriptional activity appears to depend upon ZAP-70/Syk kinases and to be negatively regulated by c-Cbl.

A critical role for p59(fyn) in CD2-based signal transduction

TCR- but not CD2-triggered IL-2 production is p56(lck) dependent. To test the hypothesis that p59(fyn), a second src-family protein tyrosine kinase (PTK) expressed in T lymphocytes, might be an essential upstream component of the CD2 signaling pathway, we generated human (h) CD2 transgenic (tg) fyn(+/+) and fyn(-/-) mice. Clustering of hCD2 molecules on resting peripheral T lymphocytes results in Ca(2+) mobilization, activation of MAPK and cellular proliferation. In contrast, in the absence of p59(fyn), these CD2-initiated activities are markedly reduced, while TCR-triggered proliferation is unaffected. Several CD2 pathway components regulated by p59(fyn) have been identified including phospholipase C-gamma1 (PLC-gamma1), Vav, protein kinase C-theta isoform (PKC-theta), docking protein (Dok), focal adhesion kinase (FAK) and Pyk2. Decreased inducible PKC-theta catalytic activity and Vav phosphorylation likely account for diminished p38 and JNK activation in hCD2tg fyn(-/-) mice. Moreover, deficiency in fyn-dependent PLC-gamma1 catalytic activity may contribute to reduced PKC-alpha-dependent ERK activation. Of note, CD2-dependent Dok but not linker from activated T cells (LAT) tyrosine phosphorylation requires p59(fyn). Furthermore, that FAK and Pyk2 are target substrates implies that p59(fyn) may be an important regulator of T cell adhesion as well. Collectively, these data identify p59(fyn) as a key PTK in CD2-mediated activation of mature T lymphocytes.

Priming of CD2-induced p62Dok tyrosine phosphorylation by CD3 in Jurkat T cells

T lymphocyte activation is triggered through the CD3-TCR complex or the CD2 molecule. Beside common biochemical events, we previously showed that a 62-kDa protein associated with PLCgamma-1 and p21RasGAP was specifically tyrosine phosphorylated after CD2 stimulation in Jurkat T cells. We demonstrated here that it was identical to p62Dok, a docking protein highly phosphorylated in human chronic myelogenous leukemia cells and in murine abl-transformed B cells. Mainly, we showed that p62Dok tyrosine phosphorylation was strengthened by the functional interplay between CD3 and CD2. Primary stimulation of Jurkat cells via CD3 suppressed most of the subsequent CD2-dependent phosphorylation events, except p62Dok tyrosine phosphorylation, which was on the contrary strongly increased. Kinetic studies indicated that a short treatment with anti-CD3 was sufficient to amplify the CD2-induced tyrosine phosphorylation of p62Dok. By contrast, CD2-induced PLCgamma-1 tyrosine phosphorylation and calcium response progressively diminished. Finally, enhanced amounts of tyrosine phosphorylated p62Dok were recruited to p21RasGAP and PLCgamma-1 after CD2 stimulation in CD3-activated cells. CD3 stimulation is known to enhance CD2 avidity for its ligand and to induce the binding of the CD2AP protein to the CD2 cytoplasmic tail. Our results suggest that the CD3-TCR complex rapidly primes the CD2 pathway to activate one of its specific components, p62Dok.

Differential requirement of ZAP-70 for CD2-mediated activation pathways of mature human T cells

This study addresses the role of the tyrosine kinase ZAP-70 in CD2-mediated T cell activation. Patients lacking ZAP-70 have few mature CD8+ T cells and high numbers of CD4+ T cells that are nonfunctional upon TCR triggering. Such a patient with a homozygous deletion in the zap-70 gene that resulted in the complete absence of ZAP-70 protein expression has been identified. Expression of the tyrosine kinases Lck, Fyn, and Syk was normal. The patient's T cells were activated with two different pairs of mitogenic mAbs. CD2-induced phosphorylation of the zeta-chain and influx of Ca2+ was defective in the ZAP-70-deficient T cells, whereas CD2-induced phosphorylation of several other proteins, including Syk, was not affected. CD2-induced proliferation as well as production of TNF-alpha and IFN-gamma was abrogated in ZAP-70-deficient T cells, whereas PMA plus ionomycin induced normal activation of these cells. Together, this study shows that CD2-activation triggers ZAP-70-dependent and -independent pathways. Deletion of ZAP-70 affected CD2- and CD3-mediated proliferation and cytokine production in a similar way, suggesting that one of the different CD2 pathways converges with a CD3 pathway at or upstream of the activation of ZAP-70.

Expression and induction of costimulatory and adhesion molecules on acute myeloid leukemic cells: implications for adoptive immunotherapy

OBJECTIVE

Previously, we observed an increased recognition of malignant cells by cytotoxic T lymphocytes (CTL) when the target cells were cultured in vitro for 24 hours. In this study, we analyzed the expression of costimulatory and adhesion molecules on acute myeloid leukemia (AML) cells and determined whether 24-hour culture of the cells was associated with upregulation of these molecules. We analyzed whether this incubation period improved recognition of AML cells by CTL.

MATERIALS AND METHODS

Expression of costimulatory and adhesion molecules on leukemic blasts of 34 patients comprising each AML FAB subclassification were analyzed directly and after 24 hours of culture, and the recognition of these AML cells by an HLA-A2 restricted CTL clone was determined. Blocking studies were performed with antibodies against CD54, CD58, and CD11a.

RESULTS

Immunophenotyping showed a low expression of CD80 and CD40 and a variable CD86 expression on most AML cells. CD54 expression was generally low, CD58 expression was high, and CD11a expression was variable, with a higher expression in AML M0, M1, M4, and M5. Twenty-four hours of culture resulted in a significant upregulation of CD40, CD54, and CD58. Impaired recognition of AML cells by the HLA-A2 restricted CTL clone was enhanced 100-200% by 24 hours of preincubation of the leukemic cells. Blocking studies showed the importance of multiple adhesion molecules on the AML cells.

CONCLUSION

Low expression of multiple costimulatory and adhesion molecules on AML could be upregulated by 24 hours of culture, which was associated with increased recognition of the AML blasts by CTL. Blocking multiple adhesion molecules completely abolished CTL recognition, showing the importance of the combination of these molecules for T-cell interaction with AML.

Regulation of integrin function by T cell activation: points of convergence and divergence

Lymphocyte adhesiveness is dynamically regulated in response to conditions in the extracellular environment. One mechanism of regulation of integrin adhesion receptors involves a rapid, but transient, increase in integrin function upon T lymphocyte activation. These integrin activating signals can be initiated either via ligation of Ig superfamily members that are coupled to tyrosine kinase cascades, such as the CD3/T cell receptor, CD2, and CD28, or by G protein-coupled receptors for chemokines. Analysis of integrin activation induced by CD3/TCR, CD2 and CD28 suggests a critical role for phosphoinositide 3-OH kinase (PI 3-K). This review summarizes recent insights into PI 3-K-dependent regulation of integrin function in leukocytes, including the mechanisms by which these receptors are coupled to PI 3-K, and potential downstream effectors of PI 3-K that regulate integrin-mediated adhesion in leukocytes.

Investigational treatments in rheumatoid arthritis

The active search for new treatment modalities of established rheumatoid arthritis have created a dynamic period for rheumatology. Both promising pharmaceutical products and targeted interventions with products of the biotechnology industry are being developed. Leflunomide and the selective blockade of the cytokine tumour necrosis factor (TNF) have recently been registered in several countries and others will follow. Like all new therapeutic strategies much remains to be learned about the optimal use of these therapies and their possible limitations. The success of these interventions have shown that a complex disease such as rheumatoid arthritis that is refractory to conventional treatment can be modulated by new therapeutic strategies. This experience has also resulted in further searches for new drugs that influence those pathogenetic pathways affected by the interventions found to be effective.

Intercellular adhesion molecule-1/leukocyte function associated antigen-1 blockade inhibits alloantigen specific human T cell effector functions without inducing anergy

BACKGROUND

Intercellular adhesion molecule (ICAM-1) is important in leukocyte adhesion-dependent events and some data suggest that ICAM-1 provides T cell costimulation. We anlayzed the role of the ICAM-1 and leukocyte function associated antigen-1 (LFA-1) interaction in human T cell alloreactivity in vitro.

METHODS

Allo-antigen-induced T cell proliferation and cytotoxic T lymphocyte lytic activity were assessed by mixed lymphocyte reaction assay and 51 Chromium release assay, respectively. Immunostaining and flow cytometry were used to assess the expression of receptors on activated T cells.

RESULTS

Alloantigen-induced T cell proliferation and cytotoxic T lymphocyte activity were markedly inhibited by antibodies to ICAM-1 and LFA-1. These antibodies had to be present at the time of initial T cell receptor/antigen engagement to inhibit proliferation. Neither IL-2 nor IL-4 were involved in the observed inhibition by antibodies. Inhibition was not associated with altered cell surface expression of receptors such as CD3, CD4, ICAM-1, LFA-1, CD25, and HLA-DR however, these antibodies did impede the ability of generation of functionally active T cells. Interestingly, these antibodies inhibited soluble, but not immobilized OKT3-induced proliferation of peripheral blood leukocytes. Antibody-mediated inhibition of proliferation failed to impair the ability of T cells to subsequently proliferate in response to stimulation by the original or third party alloantigen or mobilize [Ca++]i in response to CD3 or LFA-1 receptor ligation.

CONCLUSIONS

These data demonstrate that blockade of ICAM-1/LFA-1 binding at the time of allorecognition potently blocks initial T cell effector functions that could be due to lack of effective T cell/APC engagement.

Structure of a heterophilic adhesion complex between the human CD2 and CD58 (LFA-3) counterreceptors

Interaction between CD2 and its counterreceptor, CD58 (LFA-3), on opposing cells optimizes immune recognition, facilitating contacts between helper T lymphocytes and antigen-presenting cells as well as between cytolytic effectors and target cells. Here, we report the crystal structure of the heterophilic adhesion complex between the amino-terminal domains of human CD2 and CD58. A strikingly asymmetric, orthogonal, face-to-face interaction involving the major beta sheets of the respective immunoglobulin-like domains with poor shape complementarity is revealed. In the virtual absence of hydrophobic forces, interdigitating charged amino acid side chains form hydrogen bonds and salt links at the interface (approximately 1200 A2), imparting a high degree of specificity albeit with low affinity (K(D) of approximately microM). These features explain CD2-CD58 dynamic binding, offering insights into interactions of related immunoglobulin superfamily receptors.

Functional glycan-free adhesion domain of human cell surface receptor CD58: design, production and NMR studies

A general strategy is presented here for producing glycan-free forms of glycoproteins without loss of function by employing apolar-to-polar mutations of surface residues in functionally irrelevant epitopes. The success of this structure-based approach was demonstrated through the expression in Escherichia coli of a soluble 11 kDa adhesion domain extracted from the heavily glycosylated 55 kDa human CD58 ectodomain. The solution structure was subsequently determined and binding to its counter-receptor CD2 studied by NMR. This mutant adhesion domain is functional as determined by several experimental methods, and the size of its binding site has been probed by chemical shift perturbations in NMR titration experiments. The new structural information supports a 'hand-shake' model of CD2-CD58 interaction involving the GFCC'C" faces of both CD2 and CD58 adhesion domains. The region responsible for binding specificity is most likely localized on the C, C' and C" strands and the C-C' and C'-C" loops on CD58.

Diminished IL-2 responses and alteration of CD2 expression on CD8+ T cells are associated with a lack of cytotoxic T cell responses during Theileria annulata infection

Theileria annulata is a tick-borne protozoan parasite which causes the disease bovine tropical theileriosis. In immunized or drug-treated animals, the pathogenic macroschizont stage of the parasite is destroyed by MHC class I-restricted cytotoxic T lymphocytes (CTL). Here we show that although CD8+ T cells increase greatly in number and display activation markers during an acute infection, they exhibit no killing of infected cells. During the ineffectual response, efferent lymph cells' ability to proliferate to IL-2 drops, coinciding with loss of MoAb binding to CD2 by CD8+ cells. When animals were treated with the anti-parasite drug 'Butalex', IL-2 responses, anti-CD2 antibody binding by CD8+ cells and strong CTL activity were restored within 24 h. The initial activation of CD4+ T cells by parasite-infected cells altering the IL-2 production in the draining lymph node is the likely cause of the failure of CTL responses.

Prolonged allograft survival but no tolerance induction by modulating CD28 antibody JJ319 after high-responder rat heart transplantation

BACKGROUND

Allograft rejection depends on T cell immune responses requiring antigen recognition and costimulatory signals through accessory T cell receptors, including CD28. Inhibition of CD28 signaling with a CTLA-4-immunoglobulin (Ig) fusion protein has resulted in immunosuppression and occasional T cell anergy in mouse transplant models, but not in rats. Because this approach also inhibits a potentially tolerizing signal through CTLA-4, selective blockade of CD28 ligation might induce more profound immunosuppression and transplant tolerance.

METHODS

The effects of escalating doses of the rat CD28 monoclonal antibody JJ319 on allograft survival were studied after vascularized heterotopic heart transplantation in a high responder strain combination (DA to Lewis). CD28 antigen modulation and circulating antibody levels were monitored by flow cytometry.

RESULTS

CD28 antibody JJ319 markedly prolonged cardiac graft survival compared with untreated controls (7 days, range: 6-8). A strictly dose-dependent increase in median graft survival time was demonstrated with a maximum of 36 days (range: 30-40; p <0.001) after the administration of 8 x 1 mg JJ319 i.p. (days -1 to +6 before/after transplantation). However, indefinite graft survival and tolerance could not be induced by JJ319 treatment. At the maximal dose, flow cytometry showed complete down modulation of the CD28 receptor for 10-14 days without T cell depletion in close temporal relation to antibody presence in serum. In vitro, CD28-modulated T cells showed significantly reduced responses to activation.

CONCLUSIONS

CD28 antibody JJ319 induces profound immunosuppression after rat heart transplantation, however without development of transplant tolerance. The underlying mechanism seems to be receptor modulation during primary alloantigen recognition. While still potentially applicable clinically, there are no qualitative or quantitative differences to the treatment with CTLA-4/lg or the blockade of CD2 or LFA-1, as reported elsewhere. Thus, a CD28-modulating approach seems not to allow therapeutic exploitation of a tolerizing signal delivered by CTLA-4 but may still be clinically applicable, especially in combined immune interventions.

The Role of CD80, CD86, and CTLA4 in Alloimmune Responses and the Induction of Long-Term Allograft Survival

Blocking the interaction of the CD28 costimulatory receptor with its ligands, CD80 and CD86, inhibits in vivo immune responses, such as allograft rejection, and in some instances induces tolerance. Previously, we found that CTLA4Ig, which blocks the CD28/CTLA-4 (CD152) ligands CD80 and CD86, can be used to induce transplantation tolerance to vascularized allografts. Recent data suggest that an intact CD152-negative signaling pathway is essential for induction of tolerance to nominal Ags. Here, we show that blockade of CD152 using an anti-CD152 mAb at the time of transplantation prevents the induction of long-term allograft survival by agents that target CD80 and CD86. In contrast, CD152 signals are not required for the maintenance of established graft survival. We also report for the first time that blockade of CD86 alone can induce long-term graft survival. This requires that anti-CD86 mAb is given on the day of transplantation and also depends upon an intact CD152 pathway. This result, plus experiments using CD80-deficient mice, suggests a dominant role for CD80 molecules on donor cells as the relevant ligand for CD152. We additionally find that blockade of CD152 at the time of transplantation does not interfere with the effectiveness of anti-CD154 mAbs, suggesting distinct mechanisms for inhibition of graft rejection by blocking the CD28 vs CD154 pathways.

A cdc15-like adaptor protein (CD2BP1) interacts with the CD2 cytoplasmic domain and regulates CD2-triggered adhesion

A human CD2 cytoplasmic tail-binding protein, termed CD2BP1, was identified by an interaction trap cloning method. Expression of CD2BP1 is restricted to hematopoietic tissue, being prominent in T and natural killer (NK) cells, with long (CD2BP1L) and short (CD2BP1S) variants arising by alternative RNA splicing. Both CD2BP1 molecules are homologous to Schizosaccharomyces pombe cdc15, and include a helical domain, variable length intervening PEST sequence and C-terminal SH3 domain. Although the CD2BP1 SH3 domain binds directly to the CD2 sequence, KGPPLPRPRV (amino acids 300-309), its association is augmented markedly by the CD2BP1 N-terminal segment. Upon ligand-induced clustering of surface CD2 molecules, CD2BP1 redistributes from a cytosolic to a surface membrane compartment, co-localizing with CD2. In turn, CD2-stimulated adhesion is downregulated by CD2BP1, apparently through coupling of the protein tyrosine phosphatase (PTP)-PEST to CD2. These findings offer the first molecular view into the control processes for T cell adhesion.

Identification of a proline-binding motif regulating CD2-triggered T lymphocyte activation

An intracellular protein termed CD2 binding protein 2 (CD2BP2), which binds to a site containing two PPPGHR segments within the cytoplasmic region of CD2, was identified. Mutagenesis and NMR analysis demonstrated that the CD2 binding region of CD2BP2 includes a 17-aa motif (GPY[orF]xxxxM[orV]xxWxxx GYF), also found in several yeast and Caenorhabditis elegans proteins of unknown function. In Jurkat T cells, over-expression of the isolated CD2BP2 domain binding to CD2 enhances the production of interleukin 2 on crosslinking of CD2 but not the T cell receptor. Hence, a proline-binding module distinct from SH3 and WW domains regulates protein-protein interactions.

Phenotype, growth regulation and cytokine transcription in Ovine Herpesvirus-2 (OHV-2)-infected bovine T-cell lines

The causal agent of sheep-associated malignant catarrhal fever (MCF), Ovine Herpesvirus-2 (OHV-2), can be propagated in IL-2-dependent lymphoblastoid cell lines derived from diseased cattle and deer providing a useful model for the investigation of the pathogenesis of MCF. In this study, five interleukin-2 (IL-2)-dependent cell lines were established from affected cattle to examine their growth regulation and cytokine transcription. All cell lines expressed CD2, CD5 and CD25. Three of the cell lines were CD4+ and one CD8+, whereas one cell was of mixed CD4 and CD8 phenotye. The growth of these cell lines was reduced when cultured with antibody against CD25, the IL-2 receptor alpha subunit. All cell lines showed a lack of response to Con A and their cell growth was inhibited by Cyclosporin A which is known to inhibit cytokine promoters. It was decided therefore, to examine the cell lines for the presence of mRNA of different cytokines. The results showed that the cell lines transcribed message for IFNgamma, TNFalpha, IL-4 and IL-10 whereas no mRNA for IL-2 or IL-1beta was detected. In conclusion, the OHV-2-immortalised cell lines resemble anergic T-cells which may be activated giving rise to the characteristic lesions of MCF.

The role of clonal anergy in the avoidance of autoimmunity: inactivation of autocrine growth without loss of effector function

Exposure of mature CD4+ T cells in the peripheral immune system to peptide-antigen/MHC complexes in the absence of a threat of infection induces tolerance to the antigen as a result of both a decreased clonal frequency (peripheral deletion) and the induction of proliferative unresponsiveness (clonal anergy) in the survivors. Interestingly, Th 1-like effector functions are not automatically blocked after the development of clonal anergy. Thus, anergic T cells have the capacity to mediate Th 1-like helper activities if allowed to accumulate to high frequency. In this article, we examine those factors important to the development of tolerance versus immunity against protein antigen, and speculate on the relationship that exists between effective peripheral tolerance induction and the avoidance of autoimmune disease.

A p56lck-independent pathway of CD2 signaling involves Jun kinase

The p56 Src family non-receptor tyrosine kinase has been shown to be critical for T lymphocyte differentiation and activation. Hence in the absence of p56, T cell receptor triggered activation does not occur. We now provide evidence for a CD2-based signaling pathway which, in contrast to that of the T cell receptor, is independent of p56. CD2-mediated interleukin-2 production occurs via activation of Jun kinase in cell lines lacking p56. Jun kinase then facilitates the binding of c-Jun/c-Fos heterodimers to the AP-1 consensus site and the subsequent transcriptional activity of the interleukin-2 promoter. These data elucidate differences between TCR and CD2 signaling pathways in the same T cells.

Human Endothelial Cells Effectively Costimulate Cytokine Production by, But Not Differentiation of, Naive CD4+ T Cells

We compared costimulatory signals provided by human endothelial cells (ECs) to those provided by conventional bone marrow-derived APCs, i.e., peripheral blood-adherent mononuclear cells (PBAMCs), by measuring their effects on cytokine production by naive or memory CD4+ T cells stimulated by PHA. In these assays, ECs effectively costimulate secretion of IL-2, IFN-γ, and IL-4 from both naive and memory CD4+ T cells, quantified by ELISA or intracellular cytokine staining. ECs, which lack B7 molecules, use predominantly leukocyte-function associated Ag 3 (LFA-3) to provide costimulation. ECs are comparable to or better than PBAMCs, which use both the LFA-3 and B7 molecules, at costimulating IL-2 and IL-4 production. ECs are less effective than PBAMCs at costimulating IFN-γ production by naive T cells. ECs do not secrete IL-12, and addition of exogenous IL-12 enables ECs to costimulate IFN-γ at a level comparable to that observed with PBAMCs. ECs do not promote differentiation of naive T cells to Th1-like cells, whereas PBAMCs do. Again, addition of exogenous IL-12 enables ECs to do so. Transfection of ECs to express B7-1 or B7-2 is less effective than IL-12 supplementation for restoring these responses. These experiments suggest that a deficiency in costimulation due to lack of B7 molecule expression does not fully explain the inability of ECs to activate resting naive CD4+ T cells.

Association of p59(fyn) with the T lymphocyte costimulatory receptor CD2. Binding of the Fyn Src homology (SH) 3 domain is regulated by the Fyn SH2 domain

Human CD2 is a 50-55-kDa cell surface receptor specifically expressed on the surface of T lymphocytes and NK cells. Stimulation of human peripheral blood T cells with mitogenic pairs of anti-CD2 monoclonal antibodies (mAbs) is sufficient to induce interleukin-2 production and T cell proliferation in the absence of an antigen-specific signal through the T cell receptor. CD2 has been shown previously to associate physically with the Src family protein-tyrosine kinases p56(lck) and p59(fyn). We now report that stimulation of T cells with mitogenic pairs of anti-CD2 mAbs enhanced the association of the Fyn polypeptide with the CD2 complex, whereas stimulation with single anti-CD2 mAb had minimal effect. Using glutathione S-transferase (GST) fusion proteins, we found that CD2 bound to the Src homology (SH) 3 domain of Fyn. Interestingly, the CD2-Fyn association was negatively regulated by the Fyn SH2 domain; CD2 bound poorly to GST fusion proteins expressing both the SH2 and SH3 domains of Fyn. However, the inhibitory effect of the Fyn SH2 domain on binding of the Fyn SH3 domain to CD2 was relieved by peptides containing a phosphorylated YEEI sequence that bound directly to the Fyn SH2 domain. In addition, we found that the ability of the Fyn SH2 domain to precipitate tyrosine-phosphorylated proteins, including the CD3zeta chain, was enhanced after T cell stimulation with mitogenic pairs of CD2 mAbs. Finally, overexpression of a mutated Fyn molecule, in which the ability of the Fyn SH2 domain to bind phosphotyrosine-containing proteins was abrogated, inhibited CD2-induced transcriptional activation of the nuclear factor of activated T cells (NFAT), suggesting a functional involvement of the Fyn SH2 domain in CD2-induced T cell signaling. We thus propose that stimulation through the CD2 receptor leads to the tyrosine phosphorylation of intracellular proteins, including CD3zeta itself, which in turn bind to the Fyn-SH2 domain, allowing the direct association of the Fyn SH3 domain with CD2 and the initiation of downstream signaling events.