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 blood 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 emigrants. These cells migrate directly from the bursal follicular cortex to the periphery, express the LT2 antigen, and are proposed to represent a diversified repertoire of B cell specificities that have emigrated from the bursa in the absence of interaction with environmentally derived 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 emigration from the bursa. This population may represent cells that have undergone positive selection in the bursa by interacting with environmentally derived antigens and should, therefore, contain a restricted repertoire of B cell specificities. The third population, about 5% of blood B cells, were short-lived cells that represent the progeny of postbursal B cell production. The frequency of these cells progressively increases with time, taking over from the short-lived bursal emigrants as the bursa involutes, likely as a diversified repertoire of B cell specificities.