SURVIVORS OF BURSAL B-CELL PRODUCTION AND EMIGRATION

The bursa of Fabricius is the site of primary B cell generation in the chicken. Analysis of the rate of bursal cell emigration demonstrated that about 1% of the blood B cell pool was replaced per hour by bursal emigrants. Surgical bursectomy distinguished three populations of blo od B cells. About 60% of blood B cells in 3-wk-old chickens were short lived, with a lifespan of 2 to 3 d, and included most bursal emigrant s. These cells migrate directly from the bursal follicular cortex to t he periphery, express the LT2 antigen, and are proposed to represent a diversified repertoire of B cell specificities that have emigrated fr om the bursa in the absence of interaction with environmentally derive d antigens in the follicular medulla. About 35% of blood B cells were much longer lived cells, having a lifespan exceeding 2 wk. These cells represent about 10% of bursal emigrants, which do not express the LT2 antigen and do not divide in the peripheral blood following emigratio n from the bursa. This population may represent cells that have underg one positive selection in the bursa by interacting with environmentall y derived antigens and should, therefore, contain a restricted reperto ire of B cell specificities. The third population, about 5% of blood B cells, were short-lived cells that represent the progeny of postbursa l B cell production. The frequency of these cells progressively increa ses with time, taking over from the short-lived bursal emigrants as th e bursa involutes, likely as a diversified repertoire of B cell specif icities.