Linear differentiation of cytotoxic effectors into memory T lymphocytes

Following the development of a CD4 T cell response in vivo: from activation to memory formation

The in vivo differentiation of CD4 T cells from naive to memory cells was followed after their adoptive transfer together with syngeneic dendritic cells into MHC mismatched adoptive hosts lacking lymphocytes and NK cells. Functional and molecular changes were measured as the antigenic stimulus, provided by the cotransferred dendritic cells, disappeared. Memory cells as opposed to effector cells show an inversion in the relative expression of Bcl-2 family members in favor of antiapoptotic molecules, and compared with naive cells they have an increased ratio of bcl-xL to bcl-2. They differ qualitatively from naive T cells, suggesting that accelerated CD4 memory responses can occur without the need for increased frequencies of specific T cells.

Cytotoxic T lymphocyte antigen-4 (CTLA-4) regulates primary and secondary peptide-specific CD4(+) T cell responses

CTLA-4-deficient mice develop a fatal lymphoproliferative disorder, characterized by polyclonal expansion of peripheral lymphocytes. To examine the effect of restricting the CD4(+) TCR repertoire on the phenotype of CTLA-4-deficient mice and to assess the influence of CTLA-4 on peptide-specific CD4(+) T cell responses in vitro, an MHC class II-restricted T cell receptor (AND TCR) transgene was introduced into the CTLA-4(-/-) animals. The expression of the AND TCR transgene by CD4(+) T cells delays but does not prevent the lymphoproliferation in the CTLA-4(-/-) mice. The CD4(+) T cells become preferentially activated and expand. Interestingly, young AND TCR(+) CTLA-4(-/-) mice carrying a null mutation in the rag-1 gene remain healthy and the T cells maintain a naive phenotype until later in life. We demonstrate that CTLA-4 regulates the peptide-specific proliferative response generated by naive and previously activated AND TCR(+) RAG(-/-) T cells in vitro. The absence of CTLA-4 also augments the responder frequency of cytokine-secreting AND TCR(+) RAG(-/-) T cells. These results demonstrate that CTLA-4 is a key regulator of peptide-specific CD4(+) T cell responses and support the model that CTLA-4 plays a differential role in maintaining T cell homeostasis of CD4(+) vs. CD8(+) T cells.