Relaxed negative selection in germinal centers and impaired affinity maturation in bcl-xL transgenic mice.

The role of apoptosis in affinity maturation was investigated by determining the affinity of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific antibody-forming cells (AFCs) and serum antibody in transgenic mice that overexpress a suppressor of apoptosis, Bcl-xL, in the B cell compartment. Although transgenic animals briefly expressed higher numbers of splenic AFCs after immunization, the bcl-xL transgene did not increase the number or size of germinal centers (GCs), alter the levels of serum antibody, or change the frequency of NP-specific, long-lived AFCs. Nonetheless, the bcl-xL transgene product, in addition to endogenous Bcl-xL, reduced apoptosis in GC B cells and resulted in the expansion of B lymphocytes bearing VDJ rearrangements that are usually rare in primary anti-NP responses. Long-lived AFCs bearing these noncanonical rearrangements were frequent in the bone marrow and secreted immunoglobulin G(1) antibodies with low affinity for NP. The abundance of noncanonical cells lowered the average affinity of long-lived AFCs and serum antibody, demonstrating that Bcl-xL and apoptosis influence clonal selection/maintenance for affinity maturation.

Evidence for repression of IL-2 gene activation in anergic T cells.

The induction of clonal anergy in a T cell inhibits IL-2 secretion because of the development of a proximal signal transduction defect. Fusion of anergic murine T cells to human Jurkat T leukemia cells and formation of heterokaryons failed to result in a complementation of this signaling defect and restoration of murine IL-2 mRNA inducibility. Instead, signal transduction to the human IL-2 gene became disrupted. Heterokaryons formed by the fusion of anergic murine T cells to normal murine T cells also failed to accumulate intracellular IL-2 protein in response to stimulation either with the combination of CD3 and CD28 mAbs or with ionomycin plus a protein kinase C-activating phorbol ester. The results argue against a loss-of-function signaling defect as the sole basis for clonal anergy induction and document the presence of a dominant-acting repressor molecule that inhibits signal transduction to the IL-2 gene within viable anergic T cells.

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.

Impaired lymphokine secretion in anergic CD4+ T cells leads to defective help for B cell growth and differentiation.

The ability of anergic helper T cells to interact with resting B cells was examined in vitro. B cell growth and differentiation in cocultures were found to be dependent on the expression of CD40 ligand (CD40L) on the cloned T cells, and the expression of this molecule was only marginally blocked by the induction of anergy. In contrast, secretion of IL-3, IL-4, IL-5, and IL-6 within the cocultures was found to be significantly reduced following the induction of anergy, and this correlated with the development of a 3- to 10-fold decrease in the ability of the T cells to induce B cell proliferation and IgG secretion. In contrast to the B cells, the activation of the T cells in these cocultures did not result in proliferation; thus, the effects of T cell anergy observed on the B cell responses were independent of an ability of clonal anergy to block T cell clonal expansion. In one T cell clone (E6), lymphokine production was reduced in part because of an increased propensity to undergo apoptosis; nevertheless, two other clones (A.E7 and 16B.2) showed no reduced viability after anergy induction. Finally, the addition of rIL-2 to the anergic T cells significantly improved their helper activity relative to control cells; this was associated with a partial reversal of the IL-3, - 4, and -5 production defects. Therefore, clonal anergy can interfere with the delivery of helper lymphokines by T cells, resulting in a decreased capacity to stimulate the growth and differentiation of B cells.