Cytokine receptor signaling and costimulatory receptor signaling play distinct roles in T cell activation. Nonetheless, deficiencies in either of these pathways lead to seemingly similar phenotypes of impaired T cell homeostasis. A dramatic expansion of CD4(+) peripheral T cells with an activated phenotype has been observed in both Janus kinase (Jak) 3-deficient and CTLA-4-deficient mice. Despite these similarities, the mechanisms driving T cell expansion may be distinct. To address this possibility, we examined the TCR repertoire of peripheral T cells in Jak3(-/-) and CTLA-4(-/-) mice using complementarity-determining region 3 spectratype analysis. Interestingly, a restricted and highly biased TCR repertoire was observed in the Jak3(-/-) T cells, strongly supporting a role for foreign Ag in the activation and expansion of these cells. In contrast, CTLA-4(-/-) T cells had a diverse and unbiased TCR repertoire, suggestive of a universal, Ag-independent mechanism of activation and expansion. These findings provide insight into the diverse mechanisms controlling T cell homeostasis.

CD8 T cells exist in a dynamic network whose repertoire remains static in the absence of infection but changes in the presence of foreign antigens. Individuals each have unique T-cell repertoires that continually evolve in the presence of antigen and of cross-reactive heterologous antigens, and homeostatic forces drive deletions in T-cell memory pools to accommodate the entry of new memory cells into a finite immune system.

The role of negatively signaling NK cell receptors of the Ly49 family on the specificity of the acute CD8(+) cytotoxic T-lymphocyte (CTL) response was investigated in lymphocytic choriomeningitis virus (LCMV)-infected C57BL/6 mice. Activated CD8(+) T cells coexpressing Ly49G2 expanded during LCMV infection, and T-cell receptor analyses by flow cytometry and CDR3 spectratyping revealed a unique polyclonal T-cell population in the Ly49G2(+) fraction. These cells lysed syngeneic targets infected with LCMV or coated with two of three LCMV immunodominant peptides examined. Transfection of these sensitive targets with H2D(d), a ligand for Ly49G2, inhibited lysis. This was reversed by antibody to Ly49G2, indicating effective negative signaling. LCMV characteristically induces an anti-H2(d) allospecific T-cell response that includes T-cell clones cross-reactive between allogeneic and LCMV-infected syngeneic targets. The CD8(+) Ly49G2(+) population mediated no allospecific killing, nor was any NK-like killing observed against YAC-1 cells. This study shows that CD8(+) Ly49G2(+) cells participate in the virus-induced CTL response but lyse a more restricted range of targets than the rest of the virus-induced CTL population.

Using a variety of techniques, including limiting dilution assays (LDA), intracellular IFNgamma assays, and Db-IgG1 MHC dimer staining to measure viral peptide-specific T cell number and function, we show here that heterologous virus infections quantitatively delete and qualitatively alter the memory pool of T cells specific to a previously encountered virus. We also show that a prior history of a virus infection can alter the hierarchy of the immunodominant peptide response to a second virus and that virus infections selectively reactivate memory T cells with distinct specificities to earlier viruses. These results are consistent with a model for the immune system that accommodates memory T cell populations for multiple pathogens over the course of a lifetime.

Numerous studies have examined T cell receptor (TCR) usage of selected virus-specific T cell clones, yet little information is available regarding the stability and diversity of TCR repertoire usage during viral infections. Here, we analyzed the Vbeta8.1 TCR repertoire directly ex vivo by complementarity-determining region 3 (CDR3) length spectratyping throughout the acute lymphocytic choriomeningitis virus (LCMV) infection, into memory, and under conditions of T cell clonal exhaustion. The Vbeta8 population represented 30-35% of the LCMV-induced CD8(+) T cells and included T cells recognizing several LCMV-encoded peptides, allowing for a comprehensive study of a multiclonal T cell response against a complex antigen. Genetically identical mice generated remarkably different T cell responses, as reflected by different spectratypes and different TCR sequences in same sized spectratype bands; however, a conserved CDR3 motif was found within some same sized bands. This indicated that meaningful studies on the evolution of the T cell repertoire required longitudinal studies within individual mice. Such longitudinal studies with peripheral blood lymphocyte samples showed that (a) the virus-induced T cell repertoire changes little during the apoptosis period after clearance of the viral antigens; (b) the LCMV infection dramatically skews the host T cell repertoire in the memory state; and (c) continuous selection of the T cell repertoire occurs under conditions of persistent infections.