Homeostasis and effector function of lymphopenia-induced "Memory-Like" T cells in constitutively T cell-depleted mice

Naive T lymphocytes acquire a phenotype similar to Ag-experienced memory T cells as a result of proliferation under lymphopenic conditions. Such "memory-like" T (T-ML) Cells constitute a large fraction of the peripheral T cell pool in patients recovering from T cell ablative therapies, HIV patients under highly active antiretroviral therapy, and in the elderly population. To generate a model that allows characterization of T-ML cells without adoptive transfer, irradiation, or thymectomy, we developed genetically modified mice that express diphtheria toxin A under control of a loxP-flanked stop cassette (R-DTA mice). Crossing these mice to CD4Cre mice resulted in efficient ablation of CD4 single-positive thymocytes, whereas double-positive and CD8 single-positive thymocytes were only partially affected. In the periphery the pool of naive (CD44(low)CD62L(high)) T cells was depleted. However, some T cells were resistant to Cre activity, escaped deletion in the thymus, and underwent lymphopenia-induced proliferation resulting in a pool of T-ML cells that was similar in size and turnover to the pool of CD44(high)CD62L(low) "memory phenotype" T cells in control mice. CD4Cre/R-DTA mice remained lymphopenic despite the large available immunological "space" and normal Ag-induced T cell proliferation. CD4Cre/R-DTA mice showed a biased TCR repertoire indicating oligoclonal T cell expansion. Infection with the helminth Nippostrongylus brasiliensis resulted in diminished effector cell recruitment and impaired worm expulsion, demonstrating that T-ML cells are not sufficient to mediate an effective immune response.