T cell dependent B cell differentiation in the chicken.

Transplantation of bursa cells into surgically thymectomized (STx), cyclophosphamide(CY)-treated and X-irradiated (X) newly hatched chicks resulted in a morphological and functional reconstitution of the bursa-dependent immune system only when the transplanted bursa cells were supplemented with thymus cells. Without supplementation with thymus cells, morphological restoration of the bursa, antibody formation to sheep red blood cells and Brucella abortus, and germinal center formation remained deeply deficient. Bursa cells alone induced a full reconstitution of the bursa-dependent immunity in non-thymectomized. CY-treated and X-irradiated cell recipients. The incomplete restoration of STx-CY-X chickens after transplantation of bursa cells was not due only to lack of T helper cells, as indicated by the incomplete restoration of the bursal morphology and of antibody formation against Brucella abortus, a thymus-independent antigen. It is concluded that T cells are necessary for the B cell differentiation in the bursa of Fabricius. The mechanism behind this phenomenon remains unclear but our results indicate that a complete T cell function is not a prerequisite for this, since STx-CY-X birds reconstituted with bursa and thymus cells together did not have their mitogen (PHA, Con A) responses normalized.