Incidence of the insulin-dependent Diabetes mellitus in BB rats: their genetic heterogeneity and susceptibility to infection.

Incidence and onset of the IDDM were studied in progeny of different mating types and generations of the partially inbred BB/K and randombred BB/DK substrains. The first two generations of BB/K rats showed a high incidence (75-80%) of IDDM which corresponded to their relative genetic homogeneity (homozygotes for haemoglobin and esterase-1 loci). But the incidence of IDDM did not depend on the type of parental mating, probably due to residual genetic heterogeneity (for two non-RT1 antigens and three biochemical traits). The randombred BB/DK strain showed a relatively low incidence (11.4-16.7%) of IDDM, and its later onset. Mycoplasma infection lowered drastically the IDDM incidence in the 3rd generation of BB/K rats. This infection caused most probably a pronounced distortion of the segregation ratio in the (BB/K x LEW)F2 hybrids. The RT1u haplotype showed a lethal or sublethal effect in the homozygous state. The BB/K substrain became extinct in the 7th generation from mycoplasma infection. Interactions between infection and development of IDDM, its polygenic nature and role of the RT1u variant haplotype are discussed.