Chickens bursectomized with mibolerone have Ig-positive cells which lack bursal cell specific antigens

The bursa anlage failed to develop in chicken embryos injected with 5 micrograms mibolerone on the 5th day of embryonation. Humoral immune responses in treated birds were studied by challenging them with sheep red blood cells (SRBC) and killed Brucella abortus (B. abortus) at 4, 5, 6 and 7 weeks of age. Control birds responded with IgM and IgG titers to SRBC and B. abortus, whereas mibolerone-treated birds completely failed to respond to B. abortus and their responses to SRBC consisted of only IgM agglutinins. Mibolerone-treated birds lacked natural agglutinins to rabbit red blood cells. The total concentration of plasma IgG, measured by radial immunodiffusion, was diminished whereas the IgM level was not influenced by the mibolerone treatment. Mibolerone-treated birds lacked bursal cell specific antigens in the peripheral blood leucocytes (PBL) but they still had Ig-positive fluorescing cells which were fewer in number than those of controls. Splenomegaly resulting from graft vs. host reactions induced in chick embryos by PBL from mibolerone-treated birds was similar to that in control birds, indicating normal alloreactive T cells. These results suggest that IgM responses observed in mibolerone-treated birds are produced by cells from an extrabursal site, and that mibolerone can be used effectively to chemically bursectomize chickens.