Three breeding lines, originating from dogs with SLE, have been established. Two lines were initiated by mating a female with SLE with a normal male. The third line resulted from a mating of two affected dogs. Brother-to-sister matings have reached the third generation in each line. In addition, backcross and outcross matings were carried out. More than one-third of the autopsied dogs had thymic abnormalities. The commonest lesion was a lymphoid follicle; the thymus of one dog contained multiple granulomas, and in one animal a reticulum cell sarcoma of the thymus was found.

Multiple serological abnormalities, including positive LE cell tests, anti-nuclear antibodies (ANA), and rheumatoid factor, were found in the progeny. The development of ANA appeared unrelated to the incidence of positive LE cell tests. About 10% of the animals had rheumatoid factor in their serum. Control populations of dogs; including house pets; two other, unrelated lines of inbred dogs; and normal dogs housed in the same facility as the SLE colony did not have these abnormalities. The incidence of positive LE cell tests in the inbred, backcross, and outcross matings was not consistent with any conventional genetic mechanism of inheritance. It is conceivable that the results can be explained by vertical transmission of an infectious agent in a genetically susceptible individual.

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