An analysis of the response of recombinant congenic lines of chickens to RSV challenge provides evidence for further complexity of the genetic structure of the chicken MHC (B).

We found that CB (B12/B12) and CB.R1 (B12r1/B12r1) congenic chicken lines differing from each other only in the B-G region of the MHC(B), although both of them could be characterized as regressors of Rous sarcomas induced by PR-C and BH-C, are distinguishable in their response to the challenge with these two viruses by means of some parameters of tumour growth. This points further to the concept of functional and structural complexity of the B-G region. Both CB and CB.R1 lines are highly susceptible to progressive growth of sarcomas induced by BH-D virus. This suggests a crucial role of helper viruses of different antigenic subgroups used for complementation of the Bryan high-titre pseudotype (BH) of RSV in the pathogenesis of tumours in this experimental system. Furthermore, attempts were made to analyse both primary and secondary response to the challenge with different strains of RSV in the genetic model encompassing the Prague congenic lines CB (B12/B12), CC (B4/B4), CB.R1 (B12r1/B12r1), CC.R1 (B4r1/B4r1) and their F1 hybrids, and a number of backcross matings. The data led to the view that interacting genes within both B-F/L and B-G regions of the MHC(B) govern the observed hierarchy of the response to RSV challenge.