Immunologic and enzymatic studies of two breeding lines of NZB mice that differ in chromosome breakage

Two sublines of NZB/BI mice were developed by selective matings according to chromosome breakage frequencies. These sublines--HB, a line with high chromosome breakages, and LB, a line with low or normal breakage rates--were studied in regard to the age of the animals as it related to two different aspects. The first aspect was immunologic: A decreased response to T-cell mitogens was found in old NZB mice, but this response was more pronounced in HB mice. The response to the B-cell mitogen (lipopolysaccharide) was increased in both sublines as compared to that in BALB/c mice. The percentages of IgG-positive and theta-positive spleen cells were evaluated in both sublines: Some increase in IgG-positive cells was observed in the spleens of 2- to 8-month-old NZB mice and a slight decrease was seen after age 8 months. The percentage of theta-positive cells diminished according to the age of the mice, and the decrease occurred earlier in HB than in LB mice. The second aspect studied was enzymatic and concerned the levels of DNA alpha- and beta-polymerases and terminal DNA nucleotidyltransferase in the thymuses and spleens of these animals. The major finding noted was an augmentation of 100-200% in the terminal DNA nucleotidyltransferase levels in HB thymuses by comparison with LB thymuses. The levels of both polymerases were increased in spleen cells of HB mice as compared to those of LB mice.