Interferon-γ (IFNγ) is important in regulating the adaptive immune response, and most current evidence suggests that it exerts a negative (proapoptotic) effect on CD8+ T cell responses. We have developed a novel technique of dual adoptive transfer, which allowed us to precisely compare, in normal mice, the in vivo antiviral responses of two T cell populations that differ only in their expression of the IFNγ receptor. We use this technique to show that, contrary to expectations, IFNγ strongly stimulates the development of CD8+ T cell responses during an acute viral infection. The stimulatory effect is abrogated in T cells lacking the IFNγ receptor, indicating that the cytokine acts directly upon CD8+ T cells to increase their abundance during acute viral infection.

Genotyping microarrays are an important resource for genetic mapping, population genetics, and monitoring of the genetic integrity of laboratory stocks. We have developed the third generation of the Mouse Universal Genotyping Array (MUGA) series, GigaMUGA, a 143,259-probe Illumina Infinium II array for the house mouse (Mus musculus). The bulk of the content of GigaMUGA is optimized for genetic mapping in the Collaborative Cross and Diversity Outbred populations, and for substrain-level identification of laboratory mice. In addition to 141,090 single nucleotide polymorphism probes, GigaMUGA contains 2006 probes for copy number concentrated in structurally polymorphic regions of the mouse genome. The performance of the array is characterized in a set of 500 high-quality reference samples spanning laboratory inbred strains, recombinant inbred lines, outbred stocks, and wild-caught mice. GigaMUGA is highly informative across a wide range of genetically diverse samples, from laboratory substrains to other Mus species. In addition to describing the content and performance of the array, we provide detailed probe-level annotation and recommendations for quality control.

Viral infections often induce potent CD8 T cell responses that play a key role in antiviral immunity. After viral clearance, the vast majority of the expanded CD8 T cells undergo apoptosis, leaving behind a stable number of memory cells. The relationship between the CD8 T cells that clear the acute viral infection and the long-lived CD8 memory pool remaining in the individual is not fully understood. To address this issue, we examined the T cell receptor (TCR) repertoire of virus-specific CD8 T cells in the mouse model of infection with lymphocytic choriomeningitis virus (LCMV) using three approaches: (a) in vivo quantitative TCR β chain V segment and complementarity determining region 3 (CDR3) length repertoire analysis by spectratyping (immunoscope); (b) identification of LCMV-specific CD8 T cells with MHC class I tetramers containing viral peptide and costaining with TCR Vβ–specific antibodies; and (c) functional TCR fingerprinting based on recognition of variant peptides. We compared the repertoire of CD8 T cells responding to acute primary and secondary LCMV infections, together with that of virus-specific memory T cells in immune mice. Our analysis showed that CD8 T cells from several Vβ families participated in the anti-LCMV response directed to the dominant cytotoxic T lymphocyte (CTL) epitope (NP118–126). However, the bulk (∼70%) of this CTL response was due to three privileged T cell populations systematically expanding during LCMV infection. Approximately 30% of the response consisted of Vβ10+ CD8 T cells with a β chain CDR3 length of nine amino acids, and 40% consisted of Vβ8.1+ (β CDR3 = eight amino acids) and Vβ8.2+ cells (β CDR3 = six amino acids). Finally, we showed that the TCR repertoire of the primary antiviral CD8 T cell response was similar both structurally and functionally to that of the memory pool and the secondary CD8 T cell effectors. These results suggest a stochastic selection of memory cells from the pool of CD8 T cells activated during primary infection.

4-1BB (CD137) is a costimulatory molecule expressed on activated T cells and interacts with 4-1BB ligand (4-1BBL) on APCs. To investigate the role of 4-1BB costimulation for the development of primary immune responses, 4-1BBL-deficient (4-1BBL−/−) mice were infected with lymphocytic choriomeningitis virus (LCMV). 4-1BBL−/− mice were able to generate CTL and eliminate acute LCMV infection with normal kinetics, but CD8 T cell expansion was 2- to 3-fold lower than in wild-type (+/+) mice. In the same mice, virus-specific CD4 Th and B cell responses were minimally affected, indicating that 4-1BB costimulation preferentially affects CD8 T cell responses. This result contrasts with our earlier work with CD40L-deficient (CD40L−/−) mice, in which the CD8 T cell response was unaffected and the CD4 T cell response was markedly impaired. When both 4-1BBL- and B7-dependent signals were absent, CD8 T cell expansion was further reduced, resulting in lower CTL activity and impairing their ability to clear LCMV. Altogether, these results indicate that T cells have distinct costimulatory requirements: optimal CD8 responses require 4-1BBL-dependent interactions, whereas CD4 responses are minimally affected by 4-1BB costimulation, but require CD40-CD40L and B7-dependent interactions.

Although the role of CD28-B7 interaction in the activation of naive T cells is well established, its importance in the generation and maintenance of T cell memory is not well understood. In this study, we examined the requirement for CD28-B7 interactions in primary T cell activation and immune memory. Ag-specific CD8 T cell responses were compared between wild-type (+/+) and CD28-deficient (CD28^−/−) mice following an acute infection with lymphocytic choriomeningitis virus (LCMV). During the primary response, there was a substantial activation and expansion of LCMV-specific CD8 T cells in both +/+ and CD28^−/− mice. However, the magnitude of the primary CD8 T cell response to both dominant and subdominant LCMV CTL epitopes was ∼2- to 3-fold lower in CD28^−/− mice compared with +/+ mice; the lack of CD28-mediated costimulation did not lead to preferential suppression of CD8 T cell responses to the weaker subdominant epitopes. As seen in CD28^−/− mice, blockade of B7-mediated costimulation by CTLA4-Ig treatment of +/+ mice also resulted in a 2-fold reduction in the anti-LCMV CD8 T cell responses. Loss of CD28/B7 interactions did not significantly affect the generation and maintenance of CD8 T cell memory; the magnitude of CD8 T cell memory was ∼2-fold lower in CD28^−/− mice as compared with +/+ mice. Further, in CD28^−/− mice, LCMV-specific memory CD8 T cells showed normal homeostatic proliferation in vivo and also conferred protective immunity. Therefore, CD28 signaling is not necessary for the proliferative renewal and maintenance of memory CD8 T cells.