MHC dosage effects on primary immune organ development in the chicken

Immune development in vertebrates is thought to be influenced by many factors including genotype. We used the Trisomic avian model to probe for influences of the major histocompatibility complex (MHC) on the development of the primary immune organs. Chickens were produced having two, three, and four copies of the MHC-encoding microchromosome. Studies of growth, immune organ development and structure, and lymphocyte populations were performed at three post-hatching time points. Chickens with three and four MHC copies (trisomic and tetrasomic) exhibited reduced overall growth and had smaller bursae and thymuses compared to disomic controls. However, the fold reductions in immune organ weights were much greater than for body weights. Histological analysis of the immune organs revealed dramatic alterations in follicular composition in tetrasomic bursae compared to disomic controls. No obvious alterations were observed in thymic and splenic histology of tetrasomics. Studies of organ lymphocyte numbers revealed a sharp reduction in bursal and thymic lymphocyte counts of trisomic and tetrasomic chickens. Based on these results, it appears that MHC dosage modulates, in a targeted fashion, the cellularity of the primary immune organs through an extended period in differentiation of B and T lymphocytes.