Increasing the pool of cells at early T-cell developmental stages enhances thymopoiesis and is especially beneficial when T-cell production is compromised by radiation or aging. Within the immature double-negative (DN; CD4(-)CD8(-)) thymocyte subpopulation, the DN1 subset contains the most primitive cells including the rare early T-cell progenitors (ETPs). In the present study, a human MCL1 transgene, under the control of its endogenous promoter, resulted in enlargement of an undistorted thymus in C57/BL6 mice. Enlargement occurred in females but not males, being seen at 1 month of age and maintained during progression into adulthood as the thymus underwent involution. The small DN1 subset was expanded disproportionally (ETPs increasing from ∼0.016 to 0.03% of thymocytes), while more mature thymocytes were increased proportionally (1.5-fold) along with the stroma. DN1 cells from transgenic females exhibited increased viability with maintained proliferation, and their survival in primary culture was extended. Exposure of transgenic females to γ-irradiation also revealed an expanded pool of radioresistant DN1 cells exhibiting increased viability. While the viability of DN1 cells from transgenic males was equivalent to that of their non-transgenic counterparts directly after harvest, it was enhanced in culture-suggesting that the effect of the transgene was suppressed in the in vivo environment of the male. Viability was increased in ETPs from transgenic females, but unchanged in more mature thymocytes, indicating that primitive cells were affected selectively. The MCL1 transgene thus increases the viability and pool size of primitive ETP/DN1 cells, promoting thymopoiesis and radioresistance in peripubescent females and into adulthood.

Viral infection results in the generation of massive numbers of activated effector CD8؉ T cells that recognize viral components. Most of these are short-lived effector T cells (SLECs) that die after clearance of the virus. However, a small proportion of this population survives and forms antigen-specific memory precursor effector cells (MPECs), which ultimately develop into memory cells. These can participate in a recall response upon reexposure to antigen even at protracted times postinfection. Here, antiapoptotic myeloid cell leukemia 1 (MCL1) was found to prolong survival upon T cell stimulation, and mice expressing human MCL1 as a transgene exhibited a skewing in the proportion of CD8 ؉ T cells, away from SLECs toward MPECs, during the acute phase of vaccinia virus infection. A higher frequency and total number of antigen-specific CD8 ؉ T cells were observed in MCL1 transgenic mice. These findings show that MCL1 can shape the makeup of the CD8 ؉ T cell response, promoting the formation of long-term memory. IMPORTANCE During an immune response to a virus, CD8 ؉ T cells kill cells infected by the virus, and most die when the infection resolves. However, a small proportion of cells survives and differentiates into long-lived memory cells that confer protection from reinfection by the same virus. This report shows that transgenic expression of an MCL1 protein enhances survival of memory CD8 ؉ T cells following infection with vaccinia virus. This is important because it shows that MCL1 expression may be an important determinant of the formation of long-term CD8؉ T cell memory.U pon exposure to infectious agents, T cells undergo changes in gene expression that promote the generation and survival of effector cells, followed by the emergence of long-lived memory cells. The acute phase of virus infection results in the following sequence of events in CD8 ϩ T cells. A primary phase of clonal expansion generates cytolytic effector cells to facilitate elimination of the pathogen. This is followed by a contraction phase, during which a large number of potentially damaging cytotoxic effector cells undergo apoptosis. However, a number of cells survive this contraction and form the precursors of memory cells. Finally, during the memory phase, a small subset of antigen-specific CD8 ϩ T cells is maintained for an extended period, providing memory for later recall responses (1).While short-lived effector cells (SLECs) are important for the resolution of infection, memory precursor effector cells (MPECs) differentiate into the long-lived memory population (2). MPECs exhibit differences from SLECs in terms of phenotype and function (3). While both populations elaborate effector functions, MPECs have more subdued effector activity than SLECs (1, 4, 5). MPECs exhibit lower cell surface expression of the killer cell lectin-like receptor subfamily G member 1 (KLRG1) but higher expression of CD127 (IL-7R␣) (3). In contrast, SLECs exhibit higher KLRG1 but lower CD127 expression. In addition, interleukin-2 (IL-2) production is...