The immune response to an infection is determined by a number of factors, which also affect the generation of memory T cells afterwards. The immune response can also affect the stability of the pre-existing memory populations. The memory developed after an infection can influence the response to subsequent infections with unrelated pathogens. This heterologous immunity may deviate the course of disease and alter the disease outcome. The generation and stability of memory CD8 T cells and the influence of the history of infections on subsequent heterologous infections are studied in this thesis using different viral infection sequences. Previous studies using mice lacking individual immunoproteasome catalytic subunits showed only modest alterations in the CD8 T cell response to lymphocytic choriomeningitis virus (LCMV). In this study, I found that the CD8 T cell response to LCMV was severely impaired in mice lacking all three catalytic subunits of the immunoproteasome, altering the immunodominance hierarchy of the CD8 T cell response and CD8 T cell memory. Adoptive transfer experiments suggested that both inefficient antigen presentation and altered T cell repertoire contribute to the reduction of the CD8 T cell response in the immunoproteasome knockout mice. Immune responses generated during infections can reduce pre-existing memory T cell populations. Memory CD8 T cells have been shown to be reduced by subsequent heterologous infections. In this study, I re-examined the phenomenon using immune mice infected with LCMV, murine cytomegalovirus (MCMV) and vaccinia virus (VACV) in different infection sequences. I confirmed that memory CD8 T cells were reduced by heterologous infections, and showed that LCMV-specific memory CD4 T cells were also reduced by heterologous infections. Reduction of the memory CD8 T cells is thought to be the result of apoptosis of memory CD8 T cells associated with the peak of type I interferon early during infection. I showed that memory CD4 T cells were similarly driven to apoptosis early during infection; however, Foxp3+ CD4+ regulatory T cells were relatively resistant to virus infection-induced apoptosis, and were stably maintained during LCMV infection. The stability of Treg cells during viral infections may explain the relatively low incidence of autoimmunity associated with infections. The history of infections can deviate the course of disease and affect the disease outcome, but this heterologous immunity is not necessarily reciprocal. Previous studies have shown the effects of heterologous immunity during acute infections. In this thesis, I showed that the history of LCMV infection led to higher viral titers during persistent MCMV infection, caused more severe immunopathology at the beginning of infection, and reduced the number of MCMV-specific inflationary memory CD8 T cells after the period of memory inflation. In a different context of infection, the history of LCMV infection can be beneficial. LCMV-immune mice have been shown to have lower viral titers after VACV infection, but VACV-immune mice are not protected during LCMV infection. I found that memory CD8 T cells generated from LCMV and VACV infections were phenotypically different, but the differences could not explain the nonreciprocity of heterologous immunoprotection. By increasing the number of crossreactive VACV A11R198-205-specific memory CD8 T cells, however, I showed that some VACV-immune mice displayed reduced viral titers upon LCMV challenge, suggesting that the low number of potentially cross-reactive CD8 T cells in VACV-immune mice may be part of the reasons for the non-reciprocity of immunoprotection between LCMV and VACV. Further analysis deduced that both number of potentially cross-reactive memory CD8 T cells and the private specificity of memory CD8 T cell repertoire played a part in determining the outcome of heterologous infections.