Resistance to Marek's disease in chickens with recombinant haplotypes to the major histocompatibility (B) complex

Genetic resistance to Marek's disease (MD) is associated with the B-F region of the MHC. The resistance of chickens possessing either of two MHC haplotypes to challenge with different strains of MDV was examined. Chickens with serologically similar MHC recombinants BR2 and BR4 (both BF2-G23) were backcrossed for four generations to the highly inbred UCD-003 line (B17B17). Heterozygotes (B17BF2-G23) were mated to produce BR2BR2 and BR4BR4 homozygotes with 93% background gene uniformity. Both genotypes were highly resistant to GA-5 MDV, having an incidence of 0 and 8% MD for BR2BR2 and BR4BR4, respectively, whereas the incidence of MD in the UCD-003 birds was above 80%. Challenge with the very virulent RB-1B strain caused 10% and 31% MD in the BR2BR2 and BR4BR4 chickens, respectively, compared with 100% and 52% in the B17B17 (UCD-003) and B23B23 (New Hampshire 105) lines, respectively. Viremia levels at 5 and 6 d postinfection were significantly lower in BR2BR2 and B23B23 than in B17B17 genotypes.