We have studied the occurrence of two phenotypic components (pancreatic lymphocytic infiltration [PLI] of the pancreas and T lymphocytopenia) of the spontaneous insulin-dependent diabetic syndrome (IDDM) in the progeny of hybrids obtained by crossing BB diabetic rats with rats of inbred strains differing from the BB rat at the major histocompatibility complex, RT1. Both PLI and T lymphopenia were seen in animals with all three possible genotypes in both (BUF x BB) and (LEW x BB) lines. PLI was seen in all IDDM animals. T lymphopenia was strongly associated with overt IDDM in both lines (chi 2 = 22.28, p = 0.00002 and chi 2 = 19.28, p less than 0.00001). In addition, T lymphopenia was associated with PLI with and without IDDM in both lines (chi 2 = 8.32, p = 0.0039 an chi 2 = 3.95, p = 0.0467). Not all animals exhibiting PLI without overt IDDM had depressed T cells. Not all animals with T lymphopenia had PLI with or without IDDM. In both lines, the overt IDDM occurred only in animals with at least one RT1 u haplotype derived from the BB rat, confirming our previously reported association of IDDM and RT1. We interpret this evidence to suggest that the overt IDDM syndrome requires one MHC-linked gene and at least two non-MHC-linked genes, which determine susceptibility to PLI and to circulating T lymphocyte depression.

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