Antibody-forming capacity of B cell-deficient chickens reconstituted with limiting numbers of B cell precursors

To examine the antibody-forming capacity of neonatally cyclophosphamide-treated chickens reconstituted with limiting numbers of B cell precursors, we analyzed their antibody responses to six unrelated antigens. Our results demonstrate that about 10 x 10(6) bursal cells are needed in this adoptive cell transfer model to restore normal immune competence to B cell-deficient birds. From our earlier data we know that with low repopulating cell numbers (less than 10 x 10(6) cells) developing bursal follicles are of clonal origin. Ten million cells repopulate about 40%, i.e. about 4 x 10(3) of the bursal lymphoid follicles. Assuming the clonal origin of the follicles these results imply that the B cell system of birds receiving this dose is derived from less than 5 x 10(3) precursor cells. At the lowest reconstituting dose (1.25 x 10(6) cells) most birds do not respond to the antigens studied. However, their B cell system is derived from only about 500 precursor cells. Because the antibody repertoire of a normal chicken was estimated to be at least 10(6) our results suggest that each precursor gives rise to a large number (greater than 200) of immunoglobulin V-region gene variants during its clonal proliferation in the bursa. Our results are thus consistent with the proposed "hyperconversion" mechanism of generation of antibody diversity in the chicken and provide quantitative data useful for estimating what such somatic modification rates might be.