Analysis of the immunoglobulin isotype expression by peripheral blood rosette forming cells in chickens.

We have analysed the nature of rosette forming cells (RFC) in peripheral blood lymphocytes (PBL) from chickens which had been immunized with SRBC 7 days previously. Although none of the RFC secreted antibody immediately after harvest, a PFC response was detected after culture in the presence of SRBC. This response was of IgG isotype and was abolished by depleting RFC from cell suspensions prior to culture. In chickens with a partial immunodeficiency (produced by bursectomy 3 days post-hatching) RFC counts and IgG antibody levels were suppressed during the primary response but the memory response was unimpaired when tested 4 weeks later; thus, RFC appear to be the precursors of antibody secreting but not memory cells. The normal levels of IgM haemagglutinins found in bursectomized chickens suggested that peripheral blood RFC constitute part of the maturation pathway of IgG but not IgM antibody producing cells. Incubation of primed PBL with anti-M1 (IgM) allotype sera inhibited rosette formation. With PBL from M1/M1 heterozygous chickens, only 50% RFC inhibition was achieved by either anti-M1 or anti-M1 serum. This result, when interpreted in terms of allelic exclusion, implies that M1 receptors are endogenous cell products rather than passively acquired molecules. We conclude that RFC in chicken PBL represent B cells which are committed to IgG antibody synthesis and still express a high density of IgM antigen binding receptors at an advanced stage of maturation.