Phenotypic Characteristics Associated with the Acquisition of HSV-Specific CD8 T-Lymphocyte-Mediated Cytolytic Function in Vitro

Class I MHC-restricted, HSV-1-specific CD8+ cytolytic T lymphocyte (CTL) function is rarely detected in lymphocytes isolated directly from the lymph node draining the site of infection. However, culture in vitro for 24 to 72 h in the absence of exogenous antigen results in the development of easily detectable levels of HSV-1-specific CTL effectors. The inability to detect virus-specific CTL in HSV-1-infected mice is not well understood. However, since the in vitro culture of HSV-1-immune lymphocytes results in the transition to CTL function, studies of the changes occurring to the CD8+ T cell subpopulation may provide important insights into the development of virus-specific CTL. Therefore, the phenotypic changes taking place in the CD8+ population of T cells from draining popliteal lymph nodes of HSV-1-infected C57BL/6 (B6) mice were investigated, focusing on changes in the expression of cell surface markers associated with T lymphocyte activation. The results demonstrate an increase in the percentage of CD8+ T cells expressing the activation markers CD44 and CD25 in parallel with the acquisition of HSV-specific CTL effector function. Cytolytic function was found exclusively within the CD8+ CD44hi CD25hi fraction of cells in culture, but, surprisingly, was not detectable in CD8+ CD44hi CD25lo T cells. This suggested that the acquisition of high levels of the high-affinity IL-2 receptor was closely linked to cytolytic function and may define an important developmental stage in the transition from noncytolytic to cytolytic effector cell. In support of this, CD8+ CD25hi T cells isolated from the regional lymph node exhibited direct ex vivo cytolytic function, indicating that cytolytic effector cells were present in the lymph node, but must emigrate rapidly after attaining this level of differentiation.