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

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Abstract
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 106 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 (< 10 x 106 cells) developing bursal follicles are of clonal origin. Ten million cells repopulate about 40%, i.e. about 4 x 103 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 103 precursor cells. At the lowest reconstituting dose (1.25 X l06 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 106 our results suggest that each precursor gives rise to a large number (> 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.