CD40-CD154: A perspective from type 2 immunity

The interaction between CD40 and CD154 (CD40 ligand) is central in immunology, participating in CD4⁺ T cell priming by dendritic cells (DC), CD4⁺ T cell help to B cells and classical macrophage activation by CD4⁺ T cells. However, its role in the Th2 side of immunology including helminth infection remains incompletely understood. Contrary to viral and bacterial stimuli, helminth products usually do not cause CD40 up-regulation in DC, and exogenous CD40 ligation drives Th2-biased systems towards Th1. On the other hand, CD40 and CD154 are necessary for induction of most Th2 responses. We attempt to reconcile these observations, mainly by proposing that (i) CD40 up-regulation in DC in Th2 systems is mostly induced by alarmins, (ii) the Th2 to Th1 shift induced by exogenous CD40 ligation is related to the capacity of such ligation to enhance IL-12 production by myeloid cells, and (iii) signals elicited by endogenous CD154 available in Th2 contexts and by exogenous CD40 ligation are probably different. We stress that CD40-CD154 is important beyond cognate cellular interactions. In such a context, we argue that the proliferation response of B-cells to IL-4 plus CD154 reflects a Th2-specific mechanism for polyclonal B-cell amplification and IgE production at infection sites. Finally, we argue that CD154 is a general immune activation signal across immune polarization including Th2, and propose that competition for CD154 at tissue sites may provide negative feedback on response induction at each site.     

CD40 ligand-positive CD8+ T cell clones allow B cell growth and differentiation

A fraction of activated CD8⁺ T cells expresses CD40 ligand (CD40L), a molecule that plays a key role in T cell-dependent B cell stimulation. CD8⁺ T cell clones were examined for CD40L expression and for their capacity to allow the growth and differentiation of B cells, upon activation with immobilized anti-CD3. According to CD40L expression, CD8⁺ clones could be grouped into three subsets. CD8⁺ T cell clones expressing high levels of CD40L (≥80% CD40L⁺ cells) were equivalent to CD4⁺ T cell clones with regard to induction of tonsil B cell proliferation and immunoglobulin (Ig) production, provided the combination of interleukin (IL)-2 and IL-10 was added to cultures. CD8⁺ T cell clones, with intermediate levels of CD40L expression (10 to 30% CD40L⁺ cells), also stimulated B cell proliferation and Ig secretion with IL-2 and IL-10. B cell responses induced by these CD8⁺ T cell clones were neutralized by blocking monoclonal antibodies specific for either CD40L or CD40. By contrast, CD40L^?? T cell clones (?5 % CD40L⁺ cells), only induced marginal B cell responses even with IL-2 and IL-10. All three clone types were able to activate B cells as shown by up-regulation of CD25, CD80 and CD86 expression. A neutralizing anti-CD40L antibody indicated that T cell-dependent B cell activation was only partly dependent on CD40-CD40L interaction. These CD40L^?? clones had no inhibitory effects on B cell proliferation induced by CD40L-expressing CD8⁺ T cell clones. Taken together, these results indicate that CD8⁺ T cells can induce B cell growth and differentiation in a CD40L-CD40-dependent fashion.     

With(out) a little help from my friends: An IL‐12/CD40L‐mediated feed‐forward loop between CD8+ T cells and DCs

CD40–CD40L interactions are important for both antigen‐dependent B‐cell differentiation and effector and memory T‐cell formation. The prevailing view is that CD40L is expressed on activated CD4⁺ T cells, which enables them to provide help to high‐affinity B cells in GCs and to license DCs for efficient induction of CD8⁺ T‐cell responses. Interestingly, CD8⁺ T cells themselves can also express CD40L and, in this issue of the European Journal of Immunology, Thiel and colleagues [Eur. J. Immunol. 2013. 43: 1511‐1517] show that CD40L expression on these cells can be part of a self‐sustaining feed‐forward loop, in which expression of CD40L is induced by IL‐12 and TCR signaling. This provides a paradigm shift in our thinking about the requirements of effector CD8⁺ T‐cell development and the role herein of CD4⁺ T cells to provide help in this process.     

Endothelial cells augment the expression of CD40 ligand on newly activated human CD4+ T cells through a CD2/LFA-3 signaling pathway

Cultured human endothelial cells (EC) increase CD40 ligand expression on polyclonally activated human peripheral blood CD4⁺ helper T cells compared to T cells activated in the absence of accessory cells or in the presence of peripheral blood adherent cells or B cells. Induction of CD40 ligand expression appears to be biphasic with early induction observable at 6 h and later induction observable at 24 h. EC cause T cells to increase CD40 ligand expression during the early phase at 6 h after activation. CD40 ligand expression is restricted to the CD4⁺ helper T cell subset of the peripheral blood T cells, even when EC are present. Blocking monoclonal antibodies to co-stimulatory molecules on EC and T cells indicate that the CD2/LFA-3 pathway, which also contributes to induction of augmented interleukin-2 (IL-2) secretion, is involved in EC-induced upregulation of CD40 ligand. Exogenous IL-2 can also increase CD40 ligand expression. However, increased IL-2 secretion in the presence of EC can not fully account for endothelial-induced CD40 ligand up-regulation as (1) the effect of exogenous IL-2 is greater at 24 h than at 6 h, whereas the opposite is true for EC; (2) the effect of saturating levels of IL-2 is considerably smaller than that of EC; and (3) blocking of IL-2 receptors does not fully inhibit endothelial effects on CD40 ligand expression. We conclude that EC provide unique co-stimulatory signals that affect the phenotype of activated CD4⁺ T cells.     

Localization in situ of the co-stimulatory molecules B7.1, B7.2, CD40 and their ligands in normal human lymphoid tissue

Functional interactions between B and T lymphocytes are known to depend on the expression of co-stimulatory molecules B7.1/CD80, B7.2/CD86 and their counter-receptors CD28 and CTLA4, as well as CD40 and its ligand CD40L. To study the role of these molecules in situ, an immunohistochemical analysis was carried out on normal human lymphoid tissue. In the germinal centers (GC), B7.1 and B7.2 were differentially expressed. In the dark zone, centroblasts were predominantly B7.1⁺, while centrocytes in the light zone were B7-2⁺, resulting in reversed gradients of both markers in GC. Follicle mantle cells were negative for B7.1 and B7.2. Macrophages and interdigitating dendritic cells (IDC) in T cell zones both expressed B7.1 and B7.2. Moreover, clusters of B7.2⁺ T cells were demonstrated in interfollicular areas. Intrafollicular CD4⁺ T cells in GC, predominantly in the apical light zone, expressed CD28 and CTLA4, as did the majority of interfollicular T cells. CTLA4 showed a striking excentric cytoplasmic staining, which was also seen on T cells activated in vitro. CD40 was expressed on all B cells and more strongly on macrophages and IDC. Moreover, small clusters of T cells in a rim outside the GC showed CD40 expression. CD40L was expressed both on intrafollicular CD4⁺ T cells as well as on T cells in T cell zones. The differential distribution of co-stimulatory molecules in different compartments of normal human lymphoid tissue in situ indicates that these interactions play a distinctive role in different stages of B cell differentiation and in the immune response.     

Allospecific CD4+ T cells retain effector function and are actively regulated by Treg cells in the context of transplantation tolerance

Although donor‐specific transfusion (DST) plus CD154 blockade represents a robust protocol for inducing transplantation tolerance, the underlying mechanisms are incompletely understood. In a murine T‐cell adoptive transfer model, we have visualized alloantigen‐specific, TCR‐transgenic for H2‐A^b/H2‐K^d_54–68 epitope (TCR75) CD4⁺ T cells with indirect allospecificity during the course of tolerance induction. Three main observations were made. First, although the majority of TCR75 CD4⁺ T cells were deleted following DST plus CD154 blockade, the surviving TCR75 CD4⁺ T cells were capable of making IL‐2, upregulating CD44, and undergoing cell division, suggesting that they were functionally active. Indeed, residual TCR75 CD4⁺ T cells reisolated from the primary recipients given DST plus CD154 blockade were fully capable of rejecting allografts upon secondary transfer. Second, in tolerant mice, TCR75 CD4⁺ T cells were not induced to express Foxp3 in the graft‐draining lymph node. TCR75 CD4⁺ T cells were also absent in accepted graft tissues in which endogenous Treg cells were enriched. Finally, DST plus CD154 blockade resulted in an abortive expansion of TCR75 CD4⁺ T cells, a process that required the presence of endogenous Treg cells. Collectively, surviving TCR75 CD4⁺ T cells are immunocompetent but kept in check by an endogenous immunosuppressive network induced by DST plus CD154 blockade.     

Differential responses of human B-lymphocyte subpopulations to graded levels of CD40-CD154 interaction

Naïve and memory B‐lymphocyte populations are activated by CD154 interaction through cell‐surface CD40. This interaction plays an important role in the regulation of the humoral immune response, and increasing evidence indicates that fine variation in CD40 binding influences B lymphocytes, macrophages and dendritic cells in murine models. Here we have investigated whether and how variations in the intensity of the CD40–CD154 interaction could contribute to differential regulation of human B‐lymphocyte populations. Proliferation and differentiation of B lymphocytes were monitored in response to graded levels of CD40 stimulation in the presence of interleukin (IL)‐2, IL‐4 and IL‐10. Our results show that the level of CD154 binding to CD40 on B lymphocytes can directly influence the evolution of CD19⁺ CD27^– and CD19⁺ CD27⁺ cell populations. Furthermore, proliferation, global expansion of CD19⁺ cells and emergence of CD38⁺⁺ CD138⁺ cells, as well as immunoglobulin G (IgG) and IgM secretion, were affected by the level of exposure of B lymphocytes to CD154. These results suggest that the CD40–CD154 interaction is more like a rheostat than an on/off switch, and its variation of intensity may play a role in the regulation of B‐lymphocyte activation following the primary and/or secondary humoral immune response.     

CD40 engagement of CD4+ CD40+ T cells in a neo-self antigen disease model ablates CTLA-4 expression and indirectly impacts tolerance

Biomarkers defining pathogenic effector T (Teff) cells slowly have been forthcoming and towards this we identified CD4⁺ T cells that express CD40 (CD4⁺CD40⁺) as pathogenic in the NOD type 1 diabetes (T1D) model. CD4⁺CD40⁺ T cells rapidly and efficiently transfer T1D to NOD.scid recipients. To study the origin of CD4⁺CD40⁺ T cells and disease pathogenesis, we employed a dual transgenic model expressing OVA_323–339 peptide as a neo‐self antigen on islet β cells and medullary thymic epithelial cells (mTECs) and a transgenic TCR recognizing the OVA_323–339 peptide. CD4⁺CD40⁺ T cells and Treg cells each recognizing the cognate neo‐antigen, rather than being deleted through central tolerance, drastically expanded in the thymus. In pancreatic lymph nodes of DO11.RIPmOVA mice, CD4⁺CD40⁺ T cells and Treg cells are expanded in number compared with DO11 mice and importantly, Treg cells remain functional throughout the disease process. When exposed to neo‐self antigen, CD4⁺CD40⁺ T cells do not express the auto‐regulatory CTLA‐4 molecule while naïve CD4⁺CD40⁺ T cells do. DO11.RIPmOVA mice develop autoimmune‐type diabetes. CD40 engagement has been shown to prevent CTLA‐4 expression and injecting anti‐CD40 in DO11.RIPmOVA mice significantly exacerbates disease. These data suggest a unique means by which CD4⁺CD40⁺ T cells thwart tolerance.     

CD40 is functionally expressed on human keratinocytes

The CD40/gp39 pathway is known to be an important feature of B/T cell collaboration leading to T cell-dependent activation, proliferation or differentiation of B cells. Additionally, CD40 is involved in the regulation of B cell survival and apoptosis. Recently, CD40 has been shown to be expressed functionally on non-hematopoietic cells, i.e. endothelial cells. Here, we demonstrate that human keratinocytes (KC) cultured in vitro express CD40 constitutively. The surface expression of CD40 is markedly up-regulated following stimulation with interferon (IFN)-γ, but not with tumor necrosis factor-α or interleukin (IL)-1β. This process is regulated at the CD40 mRNA level as demonstrated by Northern blot analysis. Furthermore, ligation of CD40 via soluble gp39, the CD40 ligand, enhances intercellular adhesion molecule (ICAM)-1 and Bcl-x up-regulation on IFN-γ-stimulated KC, but not lymphocyte function-associated antigen (LFA)-3, B7-2, HLA-DR, or Fas expression. The release of IL-8 is also induced following CD40 ligation on KC. In psoriasis, a T cell-mediated inflammatory skin disease, KC have a markedly enhanced expression of CD40. This expression co-localizes with the expression of ICAM-1, Bcl-x, and an influx of CD3⁺ T cells. These findings suggest a functional role of CD40 on KC in inflammatory skin disorders such as psoriasis and could make a therapeutic intervention by disrupting the CD40/gp39 pathway an approach to consider in these inflammatory skin diseases.     

Role of the CD40-CD40 ligand interaction in CD4+ T cell contact-dependent activation of monocyte interleukin-1 synthesis

Most studies of the induction of cytokine synthesis in monocytes have employed an exogenous triggering agent such as lipopolysaccharide. However, in nonseptic inflammatory responses (e.g. rheumatoid arthritis) monocyte activation occurs as a result of T cell-generated signals. In previous reports, we and others have demonstrated that contact-dependent T cell-generated signals are capable of contributing to macrophage activation. We have shown that plasma membranes from anti-CD3 activated purified peripheral CD4⁺ T cells (Tm^A) but not from resting CD4⁺ cells (Tm^R) induce monocytes to synthesize interleukin (IL)-1 in the absence of co-stimulatory cytokines. Studies to determine the expression kinetics of the molecule(s) unique to activated CD4⁺ T cells which interact with monocytes to induce IL-1 revealed that optimal expression occurred at 6 h post activation. This matched the previously reported kinetics of expression of CD40 ligand (CD40L) on activated peripheral T cells, implicating the CD40-CD40L interaction as a candidate for the initiator of the IL-1 signaling event. The ability of Tm^A to induce IL-1 synthesis in resting monocytes could be markedly reduced by addition of a monoclonal anti-CD40L antibody, 5c8. In addition, a monoclonal anti-CD40 IgM (BL-C4) proved dramatic in its ability to induce resting monocytes to synthesize IL-1. In summary, these results demonstrate that the CD40-CD40L interaction provides a critical component of CD4⁺ T cell contact-dependent activation of monocyte IL-1 synthesis.     

Recombinant CD30 ligand and CD40 ligand share common biological activities on Hodgkin and Reed-Sternberg cells

The CD30 ligand (CD30L) and CD40L are members of the tumor necrosis factor (TNF) protein superfamily. CD30L and CD40L are mainly expressed as membrane-bound proteins by activated T cells. CD30L and CD40L are costimulatory for T cell proliferation and activation. Further, CD40L is a critical signal for T cell-dependent activation of B cells. Primary and cultured Hodgkin and Reed-Sternberg (H-RS) cells, the neoplastic component of Hodgkin's disease (HD), express high levels of the counterreceptors CD30 and CD40. We have found that both the recombinant CD30L and CD40L enhanced interleukin (IL)-6, TNF and lymphotoxin (LT)-α release from cultured H-RS cells. In addition, CD40L, but not CD30L, induced IL-8 secretion. CD30L and CD40L seem to share some redundant biological activities involved in the deregulated secretion of cytokines known to play a central role in the clinical presentation and pathology of HD. Further, CD30L enhanced surface expression of intercellular adhesion molecule-1 (ICAM-1/CD54) on cultured H-RS cells, which is frequently overexpressed on primary H-RS cells. CD30L- and CD40L-enhanced CD54 surface expression is followed by elevated shedding of CD54, as shown by detection of elevated 82-kDa soluble (s) CD54 levels in culture supernatants after stimulation with both ligands. CD30L and CD40L share common pleiotropic biological activities on CD30⁺/CD40⁺ H-RS cells and are elements of the cytokine and cell contact-dependent activation network typical for HD, a tumor of cytokine producing cells.     

Scaffold protein JLP mediates TCR-initiated CD4+T cell activation and CD154 expression

CD4⁺ T-cell activation and its subsequent induction of CD154 (CD40 ligand, CD40L) expression are pivotal in shaping both the humoral and cellular immune responses. Scaffold protein JLP regulates signal transduction pathways and molecular trafficking inside cells, thus represents a critical component in maintaining cellular functions.Its role in regulating CD4⁺ T-cell activation and CD154 expression, however, is unclear. Here, we demonstrated expression of JLP in mouse tissues of lymph nodes, thymus, spleen, and also CD4⁺ T cells. Using CD4+ T cells from jlp-deficient and jlp-wild-type mice, we demonstrated that JLP-deficiency impaired T-cell proliferation, IL-2 production, and CD154 induction upon TCR stimulations, but had no impacts on the expression of other surface molecules such as CD25, CD69, and TCR. These observed impaired T-cell functions in the jlp-/- CD4⁺ T cells were associated with defective NF-AT activation and Ca²⁺ influx, but not the MAPK, NF-κB, as well as AP-1 signaling pathways. Our findings indicated that, for the first time, JLP plays a critical role in regulating CD4⁺ T cells response to TCR stimulation partly by mediating the activation of TCR-initiated Ca²⁺/NF-AT.     

Induction,regulation, and function of soluble TRAP (CD40 ligand) during interaction of primary CD4+ CD45RA+ T cells with dendritic cells

To assess the induction, regulation, and the relative roles of cell surface tumor necrosis factor-related activation protein (TRAP; CD40 ligand) and the soluble form of TRAP (sTRAP) in the initial phase of T cell activation, primary CD4⁺ CD45RA⁺ (naive) T cells were co-cultured with mature Langerhans' cells (mLC) in the presence of superantigen. In this cell system, TRAP was very efficiently induced in T cells at both the mRNA and protein levels. After appearing on the cell surface, TRAP was rapidly down-regulated by a mechanism triggered through interaction of TRAP with CD40 on mLC. Co-culture of T cells with mLC led to the release of sTRAP, an 18-kDa protein capable of binding to CD40. Experimental data strongly suggest that sTRAP is not released by proteolytic cleavage of TRAP on the cell surface, but is generated in an intracellular compartment. Release of sTRAP and induction of TRAP cell surface expression were found to be regulated independently. In terms of function, sTRAP cannot compete with cell surface TRAP for ligation of CD40 on mLC, indicating that sTRAP release is not a mechanism for termination of the TRAP/CD40 interaction. However, sTRAP on its own rapidly down-regulates CD40 expression on mLC and has long-lasting anti-apoptotic effects on dendritic cells. Thus, we infer from our results obtained in vitro that primary activation of CD4⁺ T cells by dendritic cells in the lymphoid tissues leads to release of sTRAP, which may act on CD40⁺ bystander cells in a cytokine-like fashion.     

The development of transplant coronary artery disease after cardiac transplantation is correlated with a predominance of CD8+ T lymphocytes in endomyocardial biopsy derived T cell cultures

Long-term survival of heart transplant recipients is limited by the development of transplant coronary artery disease (TCAD). We analysed whether the development of TCAD is correlated with the incidence of acute rejection episodes, with the formation of anti-HLA antibodies or with the composition and function of T lymphocyte cultures derived from endomyocardial biopsies. TCAD was assessed by visual analysis of annually performed coronary angiograms and defined as the presence of all vascular changes, including minor wall irregularities. One year after transplantation, 31 of the 77 patients studied had TCAD (40%). The median age and mean number of HLA mismatches in patients with or without TCAD were highly comparable. The patient groups did not differ in incidence of acute rejection episodes, nor in percentage of endomyocardial biopsies yielding T cell cultures. At 1 year after transplantation, lymphocyte cultures from 18/31 TCAD⁺ patients (58%) and 27/46 TCAD^- patients (57%) were analysed. The TCAD⁺ patients had, compared with the TCAD^- patients, a higher median percentage of CD8⁺ T cells (71%versus 25%, P= 0·06) and a lower median percentage of CD4⁺ T cells (4%versus 40%, P= 0·04). Similar differences were found in a longitudinal analysis of the culture results of endomyocardial biopsies (EMBs) obtained during the first year. The cytotoxic reactivity of the cultures against donor HLA class I or class II antigens was comparable in the two groups, although a difference in recognition of heart specific antigens remains possible. The fact that EMB-derived cultures from TCAD⁺ and TCAD^- patients differed in T cell phenotype populations gives some support to the hypothesis that cellular immunological processes are involved in the development of TCAD. However, while the median values differed, the overlap of the percentages of CD8⁺ cells in cultures from TCAD^- and TCAD⁺ patients shows that other factors besides CD8⁺ T cells also play a role.     

CD40L expression by CD4+ but not CD8+ T cells regulates antiviral immune responses in acute LCMV infection in mice

CD40‐CD40 ligand (CD40L) signaling plays multiple indispensable roles in cellular and humoral immunity. Impaired memory T‐cell responses in the absence of CD40L have been well documented, but the requirement of this interaction for efficient priming of CD8⁺ T cells especially under inflammatory conditions has been under debate. In contrast to previous publications, we report here that virus‐specific CD8⁺ T‐cell responses as well as viral clearance are affected not only in the memory but also in the effector phase in CD40L^?/? mice infected with lymphocytic choriomeningitis virus (LCMV) Armstrong strain. Interestingly, a considerable part of the LCMV‐specific effector and memory T cells consists of CD40L⁺ CD8⁺ T cells. However, deficiency of CD40L in CD8⁺ T cells did influence neither the quantity nor the quality of primary T‐cell responses in LCMV infection. Virus‐specific CD8⁺ T cells in conditional knockout mice, with a selective deletion of the CD40L in CD8⁺ T cells, were fully functional regarding cytokine production and efficient pathogen clearance. Thus, our results unambiguously demonstrate that while CD40L is critical to generate effective primary CD8⁺ T‐cell responses also under inflammatory conditions, CD40L expression by CD8⁺ T cells themselves is dispensable in acute LCMV infection.     

IL‐12‐mediated STAT4 signaling and TCR signal strength cooperate in the induction of CD40L in human and mouse CD8+ T cells

CD40L is one of the key molecules bridging the activation of specific T cells and the maturation of professional and nonprofessional antigen‐presenting cells including B cells. CD4⁺ T cells have been regarded as the major T‐cell subset that expresses CD40L upon cognate activation; however, we demonstrate here that a putative CD8⁺ helper T‐cell subset expressing CD40L is induced in human and murine CD8⁺ T cells in vitro and in mice immunized with antigen‐pulsed dendritic cells. IL‐12 and STAT4‐mediated signaling was the major instructive cytokine signal boosting the ability of CD8⁺ T cells to express CD40L both in vitro and in vivo. Additionally, TCR signaling strength modulated CD40L expression in CD8⁺ T cells after primary differentiation in vitro as well as in vivo. The induction of CD40L in CD8⁺ T cells regulated by IL‐12 and TCR signaling may enable CD8⁺ T cells to respond autonomously of CD4⁺ T cells. Thus, we propose that under proinflammatory conditions, a self‐sustaining positive feedback loop could facilitate the efficient priming of T cells stimulated by high affinity peptide displaying APCs.     

Nanovesicle-targeted Kv1.3 knockdown in memory T cells suppresses CD40L expression and memory phenotype

Ca²⁺ signaling controls activation and effector functions of T lymphocytes. Ca²⁺ levels also regulate NFAT activation and CD40 ligand (CD40L) expression in T cells. CD40L in activated memory T cells binds to its cognate receptor, CD40, on other cell types resulting in the production of antibodies and pro-inflammatory mediators. The CD40L/CD40 interaction is implicated in the pathogenesis of autoimmune disorders and CD40L is widely recognized as a therapeutic target. Ca²⁺ signaling in T cells is regulated by Kv1.3 channels. We have developed lipid nanoparticles that deliver Kv1.3 siRNAs (Kv1.3-NPs) selectively to CD45RO⁺ memory T cells and reduce the activation-induced Ca²⁺ influx. Herein we report that Kv1.3-NPs reduced NFAT activation and CD40L expression exclusively in CD45RO⁺ T cells. Furthermore, Kv1.3-NPs suppressed cytokine release and induced a phenotype switch of T cells from predominantly memory to naïve. These findings indicate that Kv1.3-NPs operate as targeted immune suppressive agents with promising therapeutic potentials.     

Synergistic OX40 and CD30 signals sustain CD8+ T cells during antigenic challenge

Prior to acquiring a memory phenotype, antigen‐activated CD8⁺ T cells need to expand and then undergo a contraction phase. Utilizing two different antigenic stimuli, we provide evidence that the tumor necrosis factor receptors OX40 and CD30 integrate synergistic signals during the expansion phase to help maintain CD8⁺ effectors. Thus, double deficiency in OX40 and CD30 leads to CD8⁺ cell loss during expansion after immunization either with OVA or with murine CMV. Following their contraction, OX40‐ and CD30‐deficient CD8⁺ T cells persist normally in CMV‐infected mice. In contrast, persistence after OVA challenge is dependent on OX40 and CD30. Collectively, our data define the important role of both OX40 and CD30 during CD8⁺ T‐cell activation, and show that long‐term CD8 persistence after contraction is regulated not only by stimulatory receptors but also by the nature of the antigen or how the antigen is presented.     

CD8+ T cells and not CD4+ T cells are hyporesponsive to CD28- and CD40L-mediated activation in HIV-infected subjects

T cell dysfunction in HIV-infected subjects could be the consequence of altered sensitivity of CD4⁺ or CD8⁺ T cells to various costimulatory signals. Therefore, we studied proliferation and cytokine production in highly purified CD8⁺ and CD4⁺ T cells from HIV-infected and HIV⁻ subjects, induced by co-activation via cell-bound CD80, CD86 and CD40 or by allo-activation. Regardless of the nature of the first and the costimulatory signal, CD8⁺ T cells from patients proliferated consistently less than controls, while responses from CD4⁺ T cells were similar in patients and controls. This phenomenon was observed after ligation of CD28 combined with anti-CD3 or phorbol myristate acetate (PMA), but also after allogeneic stimulation and after activation by CD40 and anti-CD3. Anti-CD3 combined with CD80 or CD86 induced a mixed Th1/Th2-type cytokine profile in both CD4⁺ and CD8⁺ T cells from controls, whereas anti-CD3 plus CD40 induced only low levels of Th2-type cytokines and no interferon-gamma (IFN-γ) in CD4⁺ T cells. Compared with controls, CD4⁺ T cells from patients produced slightly lower levels of IL-10 but equal amounts of IFN-γ, IL-4 and IL-5, while CD8⁺ T cells from patients produced less of all cytokines tested. In conclusion, responses of purified CD4⁺ T cells from HIV⁺ subjects to various costimulatory pathways are relatively intact, whereas CD8⁺ T cells are hyporesponsive at the level of proliferation and cytokine production. A generalized intrinsic CD8⁺ T cell failure might contribute to viral and neoplastic complications of HIV infection.