A diverse T cell repertoire is essential for a vigorous immune response to new infections, and decreasing repertoire diversity has been implicated in the age-associated decline in CD8 T cell immunity. In this study, using the well-characterized mouse influenza virus model, we show that although comparable numbers of CD8 T cells are elicited in the lung and lung airways of young and aged mice after de novo infection, a majority of aged mice exhibit profound shifts in epitope immunodominance and restricted diversity in the TCR repertoire of responding cells. A preferential decline in reactivity to viral epitopes with a low naive precursor frequency was observed, in some cases leading to “holes” in the T cell repertoire. These effects were also seen in young thymectomized mice, consistent with the role of the thymus in maintaining naive repertoire diversity. Furthermore, a decline in repertoire diversity generally correlated with impaired responses to heterosubtypic challenge. This study formally demonstrates in a mouse infection model that naturally occurring contraction of the naive T cell repertoire can result in impaired CD8 T cell responses to known immunodominant epitopes and decline in heterosubtypic immunity. These observations have important implications for the design of vaccine strategies for the elderly.

The development of T cell memory from naive precursors is influenced by molecular cues received during T cell activation and differentiation. In this study, we describe a novel role for the chemokine receptors CCR5 and CXCR3 in regulating effector CD8+ T cell contraction and memory generation after influenza virus infection. We find that Ccr5−/− Cxcr3−/− cells show markedly decreased contraction after viral clearance, leading to the establishment of massive numbers of memory CD8+ T cells. Ccr5−/− Cxcr3−/− cells show reduced expression of CD69 in the lung during the peak of infection, which coincides with differential localization and the rapid appearance of memory precursor cells. Analysis of single chemokine receptor–deficient cells revealed that CXCR3 is primarily responsible for this phenotype, although there is also a role for CCR5 in the enhancement of T cell memory. The phenotype could be reversed by adding exogenous antigen, resulting in the activation and contraction of Ccr5−/− Cxcr3−/− cells. Similar results were observed during chronic Mycobacterium tuberculosis infection. Together, the data support a model of memory CD8+ T cell generation in which the chemokine-directed localization of T cells within infected tissues regulates antigen encounter and controls the extent of CD8+ T cell activation and differentiation, which ultimately regulates effector versus memory cell fate decisions.

There are well-characterized age-related changes in the peripheral repertoire of CD8 T cells characterized by reductions in the ratio of naive:memory T cells and the development of large clonal expansions in the memory pool. In addition, the TCR repertoire of naive T cells is reduced with aging. Because a diverse repertoire of naive T cells is essential for a vigorous response to new infections and vaccinations, there is much interest in understanding the mechanisms responsible for declining repertoire diversity. It has been proposed that one reason for declining repertoire diversity in the naive T cell pool is an increasing dependence on homeostatic proliferation in the absence of new thymic emigrants for maintenance of the naive peripheral pool. In this study, we have analyzed the naive CD8 T cell repertoire in young and aged mice by DNA spectratype and sequence analysis. Our data show that naive T cells from aged mice have perturbed spectratype profiles compared with the normally Gaussian spectratype profiles characteristic of naive CD8 T cells from young mice. In addition, DNA sequence analysis formally demonstrated a loss of diversity associated with skewed spectratype profiles. Unexpectedly, we found multiple repeats of the same sequence in naive T cells from aged but not young mice, consistent with clonal expansions previously described only in the memory T cell pool. Clonal expansions among naive T cells suggests dysregulation in the normal homeostatic proliferative mechanisms that operate in young mice to maintain diversity in the naive T cell repertoire.

Previous studies of Ehrlichia chaffeensis infection in the mouse have demonstrated that passive transfer of polyclonal Abs from resistant immunocompetent mice to susceptible SCID mice ameliorated infection and disease, even when Abs were administered during established infection. To identify particular Abs that could mediate bacterial clearance in vivo, E. chaffeensis-specific mAbs were generated and administered to infected SCID mice. Bacterial infection in the livers was significantly lowered after administration of either of two Abs of different isotypes (IgG2a and IgG3). Moreover, repeated administration of one Ab (Ec56.5; IgG2a) rescued mice from an otherwise lethal infection for at least 5 wk. Both protective Abs recognized the E. chaffeensis major outer membrane protein (OMP)-1g. Further studies revealed that both Abs recognized closely related epitopes within the amino terminus of the first hypervariable region of OMP-1g. Analyses of human sera showed that E. chaffeensis-infected patients also generated serological responses to OMP-1g hypervariable region 1, indicating that humans and mice recognize identical or closely related epitopes. These studies demonstrate that OMP-specific mAbs can mediate bacterial elimination in SCID mice, and indicate that Abs, in the absence of cell-mediated immunity, can play a significant role in host defense during infection by this obligate intracellular bacterium.

We previously showed a preferential degradation and down-regulation of mitochondrial DNA and RNA in hamster fibroblasts in response to hydrogen peroxide. Subsequent studies by others demonstrated that mitochondrial DNA can stimulate immune cells as a