The effects of MHC chromosome dosage on bursal B-cell subpopulations in embryonic and neonatal chickens.

The effect of MHC dosage on the population structure of developing bursal lymphocytes was studied in the Trisomic strain of chickens. These chickens contain either 2 (disomic), 3 (trisomic), or 4 (tetrasomic) copies of the MHC-encoding chromosome. To assess the developmental profile of bursal cell populations, bursal cells were characterized in the older embryo (18 days of incubation) and at two neonatal ages (1 day and 1 week post hatch) by a number of cellular parameters. Both transient and persistent alterations in cellular composition were revealed in the bursal subpopulations of trisomics and tetrasomics (aneuploids). Aneuploid bursal cells showed increased cell surface expression of MHC class II Ia antigen, at each age, compared to normal disomic cells. Aneuploid cell populations showed an increased incidence of small cells, at each age, a decreased incidence of Ia+ and IgM+ cells during late embryogenesis, and an increased incidence of IgG+ cells in the late embryo and 1-day-old chick. Significantly fewer proliferating cells were observed in the aneuploids, as compared to disomics, in the late embryo and 1-week-old chick. Three subpopulations were observed to be persistently altered in the MHC-aneuploids: an increased pool of Ia , small, IgM+ cells; a decreased pool of Ia+, large, IgM+ cells; and a decreased pool of Ia+, large, IgM- cells. Thus, increasing MHC-chromosome dosage is accompanied by specific alterations in B-cell differentiation. The elevated surface Ia antigen expression found on developing aneuploid B cells may play a role in these alterations.