LOCUS-SPECIFIC SOMATIC HYPERMUTATION IN GERMINAL CENTER T-CELLS

SOMATIC hypermutation and affinity-driven selection of active immunogl obulin genes occur in germinal centres (GCs), resulting in the generat ion of high-affinity memory B cells(1-3). In contrast, T lymphocytes d o not require the germinal centre microenvironment to establish memory and the T-cell antigen receptor (TCR) genes, though homologous to imm unoglobulin genes, are believed to be incapable of hypermutation(5-7). Here we present direct evidence that the small population of antigen- specific T cells that are recruited into splenic GCs acquire mutations in the variable region of genes encoding TCR alpha-chains (V alpha) b ut not those of beta-chains. These locus-specific mutations reach freq uencies comparable to mutated inmunoglobulin V-H exons recovered from the same site and exhibit similar substitution biases and DNA strand p olarity. T cells bearing identical mutations appear in multiple GCs, r aising the possibility that some cells bearing mutant TCRs may re-ente r the peripheral lymphocyte pool.