Identification of a chromosomal region that is responsible for memory T cell homeostasis in a mutant mouse.

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It is well known that memory T cells proliferate faster and react to the pathogens stronger compared to naive T cells. I found a female mutant mouse carrying excess amount of memory T cells of both CD4+ and CD8+ in a SPF colony in National Jewish Medical Center, Denver, CO. The phenotype is inherited as an autosomal recessive manor. Thymus size is small (about 10% of controls) and the numbers of double positive cells reduces dramatically. T cell differentiation is partially but dominantly blocked at a step where preTCR signaling is necessary named DN3. Platelets significantly increased in serum and this result is consistent with excess amount of megakaryocytes in spleen. In addition, the number of immature B cells decreased significantly and differentiation of B cells is blocked at large preB stage where preBCR signaling is important. I performed bone marrow transplantation experiment to analyze if the mutation in lymphoid progenitor is or if it in microenvironment is necessary for the phenotype of T and B cells in the mutant mice. The mutation in lymphoid progenitor cells themselves is necessary to see the phenotype. In order to identify a DNA fragment carrying a responsible gene that induces the phenotype in the mutant mice, we employed SSPL method. We crossed B10#4 mice with NZB mice to get F2 mice and got over 100 phenotype + and -offspring. We identified 1cM DNA fragment in chromosome #2 that is responsible for the mutant and the fragment has about 40 genes according to NCBI database. Result of RT-PCR experiment for a gene in the fragment was fascinating, since splicing pattern of the gene is different between the B10#4 and control mice. I analyzed sequence of the gene and found that there were two mutations in the introns of the gene.