DIFFERENT METHODS OF BURSECTOMY INDUCE DIFFERENT EFFECTS ON LEUKOCYTEDISTRIBUTION AND REACTIVITY

Different methods of bursectomy were analysed for their capacity to de lete the B-cell component of the immune system. Bursectomy was perform ed chemically by administration of cyclophosphamide (Cy) to newly hatc hed chicks, hormonally by in ovo administration of 19-nortestosterone (T5), chemically/hormonally (CyT5) by a combination of both previous m ethods, or surgically by in ovo removal of the bursal primordium after 5 days of incubation. B-cell immune responses were effectively suppre ssed in Cy- and CyT5-treated birds. These birds lacked serum immunoglo bulin (Ig)M and IgA antibodies and had only maternal IgG. They were al so unable to generate these isotypes upon immunization. Furthermore, s urface IgM(+) B-cells of the peripheral blood and the spleen were dras tically depleted. However, a transient but distinct effect was also ob served on other cell populations. Indeed, 5 days after the last Cy adm inistration splenic and thymic size were reduced and mononuclear cells of the peripheral blood (including thrombocytes) were severely deplet ed. There was also a shift in T-cells from CD4(+) towards CD8(+) cells in 15- and 22-day-old chickens. Hormonal bursectomy was less effectiv e in removing the B-cell arm of the immune system. Peripheral blood Ig M(+) cells were less drastically lowered and in some birds, a small bu rsa, a substantial number of splenic B-cells and antigen-specific anti body responses (primarily IgM) could still be observed. No alterations in T-cell populations were observed after hormonal bursectomy. Surgic al in ovo bursectomy eliminated virtually all B-cells from the circula tion. However, because of its technical complexity and low hatchabilit y, this technique appears less practical when a substantial number of bursectomised birds are required. For immunoprotective studies, the me thod of bursectomy should thus be chosen in function of the age of the chickens under investigation. For animals older than 4 weeks, Cy- or CyT5-treatment is more efficient and reproducible for eliminating anti body-dependent protective immunity than hormonal bursectomy. For young er birds, T5-treatment seems, given the unaltered distribution of T-ce ll (sub)populations, a good compromise.