The importance of the HIV variants R5 and X4 has been shown by the difference in their predominance during different stages of HIV-1 disease. Recent in vivo studies in macaques have investigated the competition between X4 and R5 viruses in vivo. In animals dually infected with X4 and R5 SHIV, R5 dominance coincided with the development of the HIV- specific immune response. When CD8+ T cells were depleted there was an outgrowth of the X4 strain which became the dominant variant. These results suggest that an effective CD8+ T cell response preferentially controls X4 virus. Preferential control of X4 virus by CD8+ T cells may occur either due to intrinsic differences in R5 and X4 viruses or differences in the susceptibility of the target cells of these viruses to CD8+ mediated control. In specific aim #1, we will determine the role of the infected cell phenotype on the susceptibility to CD8+ T cell control. We hypothesize that either: a) CD4+ cells are less susceptible than CCR5- CD4+ cells to CD8+ mediated lysis, and therefore the tropism of X4 virus for CXCR4+ cells leads to more effective control of this strain, or b) the kinetics of infection of CCCR5- cells allows more efficient control of X4 virus because it infects these cells. To differentiate these possibilities we will examine the relative susceptibility of CCR5+ and CCR5- CD4+ T cells to lysis. The ability of CD4+ T cells infected with X4 and R5 virus to process and present peptides to CD8+ T cells will then be assessed by testing the susceptibility of cells infected with these primary viruses to CD8+ T cell killing. In specific aim #2, we will investigate the effect of CD8+ T cells on the growth of X4 and R5 in CD4+ T lymphocytes in vitro. The overall productivity of infection of X4 and R5 viruses depends on the rate of killing of infected cells and also on the rate of viral production. We have recently shown that infection of cells with X4 virus is less productive than R5 virus, so R5 virus may still produce more virions per infected cell. We will perform in vitro growth competition assays in the presence of virus-specific CD8+ T lymphocytes. By comparing the growth of X4 and R5 virus in the presence or absence of CD8+ T cell pressure we can directly address whether X4 virus is preferentially affected in a mixed (CCR5+/CCR5-) population of target cells. If X4 virus is preferentially affected by CD8+ T cells, we will then assess whether this preferential control occurs because of the virus itself, or because of the targets infected.