Impaired CD28-mediated co-stimulation in anergic T cells

Validity of the two-signal model for activation of CD28-deficient T lymphocytes: quantitative characterization of an alternative costimulatory function of dendritic cells

The observation that primary T-dependent immune responses are generated in mice lacking CD28, the only receptor definitively shown to costimulate naive T cells, has led to ambiguity as to whether costimulation is absolutely required for initiation of T-cell responses. In this report, in vitro analysis of the relationship between cell density and proliferation demonstrates that activation of CD28-/- T cells to immobilized T-cell receptor (TCR)-alpha monoclonal antibodies (MoAb) depends on costimulatory signals provided by other cells in culture and occurs only at cell densities sufficient to permit these intercellular interactions. These signals are necessary even under TCR triggering conditions that obviate the CD28 requirement. Dendritic cells (DCs) provide the necessary costimulation in vitro and prime T cells in vivo in CD28-/- mice. Single-cell and limiting dilution analyses indicate that individual T cells from normal and CD28-/- mice produce equivalent interleukin (IL)-2 in response to DCs. However, half as many T cells produce IL-2 when only the CD28-independent pathway is used. Nonetheless, CD28-/- T cells produce sufficient IL-2 to support clonal expansion comparable to that of CD28+/+ T cells, which may account for the equally robust in vivo responses initiated by DCs in normal and CD28-deficient animals.

Control of CD4 T cell fate by antigen re-stimulation with or without CTLA-4 engagement 24 h after priming

After two consecutive inoculations with Staphylococcus enterotoxin B (SEB) at 24 h intervals in vivo, CD4 T cells became anergic to the antigen challenge in vitro. Administration of anti-CTLA-4 mAb in conjunction with the second SEB inoculation 24 h after antigen priming interfered with anergy and CD4 T cells became T(h)2 cells. However, the anergy induction was not ablated when SEB and anti-CTLA-4 mAb were administered 48 or 72 h after antigen priming. Moreover, anti-CTLA-4 mAb without SEB did not interfere with anergy nor promoted the T(h)2 differentiation. T-antigen-presenting cell (APC) interaction in vitro in the presence of high doses of antigen and anti-CTLA-4 mAb induced a T(h)2-polarizing cytokine IL-6 and IL-10. IL-10 then down-modulated a T(h)1-polarizing cytokine IL-12. The results demonstrate that 24 h after the initial antigen stimulation, CD4 T cells enter the critical activation phase where antigen re-stimulation with or without CTLA-4 engagement alters the fate of the cell, anergy or differentiation respectively. Once anergy is interfered with, T(h)2-polarizing cytokines produced upon prolonged T-APC interaction favor the T(h)2 differentiation.

Increased c-Fos/activator protein-1 confers resistance against anergy induction on antigen-specific T cell

We have studied the contribution of c-Fos/activator protein-1 (AP-1) to antigen-specific T cell response with reference to T cell anergy by increasing c-Fos/AP-1 in vivo and in vitro. First, after injection of a high dose of staphylococcus enterotoxin B (SEB), clonal deletion of SEB-reactive V(beta)8(+) CD4 T cells occurred both in control B6 and H2-c-fos transgenic (fos) mice, whereas proliferation of T cells against SEB was profoundly depressed in B6 but not in fos mice. Second, the keyhole limpet hemocyanin-specific CD4 T(h)1 cell clone produced decreasing amounts of IL-2 in response to increasing amounts of concanavalin A (Con A) in vitro, whereas the decrease was less significant in the T(h)1 clones stably transfected with c-fos gene. Electrophoretic mobility shift assay with nuclear protein from the transformants showed that overexpression of the c-fos gene compensated the amounts of AP-1 in the nuclei of Con A-treated T(h)1 clones. Thus, increased c-Fos/AP-1 confers resistance against anergy induction on antigen-specific T cells.

Interleukin-10 induces a long-term antigen-specific anergic state in human CD4+ T cells

Human CD4+ T cells, activated by allogeneic monocytes in a primary mixed lymphocyte reaction in the presence of exogenous interleukin (IL) 10, specifically failed to proliferate after restimulation with the same alloantigens. A comparable state of T cell unresponsiveness could be induced by activation of CD4+ T cells by cross-linked anti-CD3 monoclonal antibodies (mAbs) in the presence of exogenous IL-10. The anergic T cells failed to produce IL-2, IL-5, IL-10, interferon gamma, tumor necrosis factor alpha, and granulocyte/macrophage colony-stimulating factor. The IL-10-induced anergic state was long-lasting. T cell anergy could not be reversed after restimulation of the cells with anti-CD3 and anti-CD28 mAbs, although CD3 and CD28 expression was normal. In addition, restimulation of anergized T cells with anti-CD3 mAbs induced normal Ca2+ fluxes and resulted in increased CD3, CD28, and class II major histocompatibility complex expression, indicating that calcineurin-mediated signaling occurs in these anergic cells. However, the expression of the IL-2 receptor alpha chain was not upregulated, which may account for the failure of exogenous IL-2 to reverse the anergic state. Interestingly, anergic T cells and their nonanergic counterparts showed comparable levels of proliferation and cytokine production after activation with phorbol myristate acetate and Ca2+ ionophore, indicating that a direct activation of a protein kinase C-dependent pathway can overcome the tolerizing effect of IL-10. Taken together, these data demonstrate that IL-10 induces T cell anergy and therefore may play an important role in the induction and maintenance of antigen-specific T cell tolerance.

Reversal of mucosal tolerance by subcutaneous administration of interleukin-12 at the site of attempted sensitization

Oral feeding of proteins causes peripheral T-cell tolerance, as revealed by reduced delayed-type hypersensitivity (DTH) reactivity after immunization. This type of tolerance can be due both to passive T-cell anergy and active immunosuppression. Using ovalbumin-fed mice we studied whether putatively immunostimulatory cytokines could break this state of mucosal tolerance. Cytokines were administered locally at the site of attempted sensitization. It was found that neither interleukin-2 (IL-2), interferon-gamma (IFN-gamma) nor granulocyte-macrophage colony-stimulating factor (GM-CSF) could restore the response to immunization. In contrast, local administration of IL-12 at the site of attempted immunization resulted in full recovery of DTH reactivity. The dichotomy between the two Th1 stimulatory cytokines IFN-gamma and IL-12 was also reflected by different effects on ovalbumin-specific antibody isotypes. Although both IFN-gamma and IL-12 downregulated serum IgG1-levels in tolerant mice, suggesting decreased ovalbumin-specific Th2 function, only local administration of IL-12 led to increased serum IgG2a levels. These results support the view that potentiation of Th1 effector function is critical for reversal of mucosal tolerance.

Long-term culture of monoclonal human T lymphocytes: models for immunosenescence?

Human monoclonal T lymphocyte populations maintained in long-term culture by intermittent reactivation via the antigen receptor and supplied with exogenous interleukin 2 manifest finite proliferative lifespans. T lymphocytes cloned from mature peripheral T cells of adult donors were constantly lost from the time point of their first isolation up to an estimated maximum of 80 population doublings (PD) for the longest lived. T lymphocytes cloned from T cell progenitors in bone marrow, on the other hand, survived for a maximum of ca. 100 PD. One facet of the functional capacity of cells derived from these two different sources was assessed by measuring their autocrine proliferation after mitogenic stimulation. For a majority of T cell clones (TCC), autocrine proliferative capacity decreased as a function of culture age, becoming absent by 50 PD for adult-derived-TCC and by 70 PD for bone marrow-derived TCC, thereby clearly occurring prior to the end of the proliferative life spans of the clones. Limiting dilution frequency analysis showed that the number of autocrine proliferative precursors within these monoclonal populations declined with age, paralleling loss of autocrine proliferative capacity in the 'bulk' clones. Of a variety of surface structures monitored during culture ageing of TCC, the density of expression of the coreceptor molecule CD28 was found to correlate with decreasing autocrine proliferative capacity in two-thirds of the clones. Thus, at least for a fraction of monoclonal human T lymphocytes, decreasing autocrine proliferative capacity, a measure of clonal expansion, may correlate with decreasing numbers of CD28 molecules expressed on the surface and therefore presumably with the strength of costimulatory signal delivered via this important coreceptor.