Characterization of CD4+ T cells in mouse bone marrow. I. Increased activated/memory phenotype and altered TCR Vβ repertoire

Abstract

A significant proportion of memory B cells home to bone marrow (BM) which is a major site of anamnestic antibody responses in mice. We hypothesized that memory T cells likewise accumulate in BM perhaps to provide help for antibody production, and that the compartment of CD4+ T cells in BM of unimmunized mice would be enriched for memory phenotype cells that might have been activated by environmental antigens. The phenotype of activated/memory CD4+ lymphocytes has been defined as CD44(hi) CD45RB(lo) CD62L-. Conversely, the phenotype of immunologically naive cells is CD44(lo) CD45RB(hi) CD62L+. Flow cytrometric analysis of tissue from normal, adult C57BL/6 mice identified 1-2% CD3+CD4+ cells in BM. Up to 40% of CD3+CD4+ cells in the BM expressed the activated/memory phenotype compared with ≤ 10% in the spleen and lymph nodes. Analysis of TCR Vβ repertoire revealed that expression of Vβ3 and Vβ7 genes was increased as much as fourfold in BM compared to the periphery; most of this increase was within the CD44(hi) T cells. The accumulation of activated/memory T cells and clonotypic expansion(s) was not seen in the BM of germ-free mice, indicating that it reflects the history of the animal's exposure to antigens. Finally, immunization of mice which express a transgenic T cell receptor specific for ovalbumin peptide resulted in appearance of antigen-specific T cells with activated/memory phenotype in the BM.