CD45R CD4 T cell subset-reconstituted nude rats: subset-dependent survival of recipients and bi-directional isoform switching

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Abstract
High and low molecular weight variants (CD45R) of the leukocyte common antigen (CD45) divide CD4 T helper cells into subpopulations which display distinct characteristics. In vitro and in vivo evidence suggested that the presence of the high molecular weight splice variants CD45RA or CD45RB distinguished naive CD4 T cells from memory T cells which underwent an irreversible switch to the low molecular weight isoform as a consequence of antigen encounter. In the rat monoclonal antibody MRC OX22 identifies an epitope on the CD45RB molecule. We investigated this proposed differentiation pathway by reconstituting athymic nude rats with highly purified OX22+ or OX22− CD4 T lymphocyte subsets obtained from the thoracic duct (TDL) of euthymic, congenic, allotype‐marked donors. Injection of CD4+CD45RB− (45R−) cells ensured long‐term survival of nude recipients; recipients of CD4+CD45R+ (45R+) cells died within 2–3 months of injection. Early after transfer (3–4 weeks) the progeny of both 45R+ and 45R− TDL were uniformly 45R−. With time (by 2 months) progeny of both parental types expressed the high molecular weight CD45RB isoform. Nude recipients of 45R− TDL always generated progeny a proportion of which bore the 45R+ phenotype; 3 months to 2 years post‐injection, 30%–50% of the donor‐derived CD4 T cells were 45R+. 45R− progeny isolated from primary recipients of either 45R− or 45R+ cells transferred into secondary nude recipients induced skin allograft rejection with equal effectiveness and also generated 45R+ offspring. The results indicated that CD4 T cell subsets switched between CD45R isoforms and that the change between high and low molecular weight expression was bi‐directional. The splice variants apparently are not lineage or maturation markers, but rather identify CD4 T cells that exist transiently in different functional states.