Immune function is generally reported to be impaired in uncontrolled diabetes mellitus. We have previously reported that very-low-density lipoproteins (VLDL) from diabetic rat serum injure endothelial cells in vitro. Thus, we wanted to determine if VLDL would also injure lymphocytes and impair the immunity of the streptozotocin-induced diabetic rat. The immunologie functions of spleen cells from normal and diabetic rats were studied using the in vitro mitogenic stimulation assay and mixed lymphocyte responses (MLR). The effects of diabetic serum and VLDL isolated from diabetic rat serum on normal spleen cells were also determined. Both the absolute peripheral blood leukocyte (PBL) and spleen cell numbers of diabetic rats were significantly decreased from those of normal rats (P < 0.02). In PBL there was a decreased percentage of lymphocytes and increased percentage of neutrophils (both P < 0.02). The mitogenic stimulation responses of spleen cells from diabetic rats were consistently lower than normal for 3 of the 4 mi-togens used. The responses of con A-stimulated lymphocytes from diabetic spleens were significantly different (P < 0.025) from normal spleen cells. The MLR appeared to be normal. The decreased mitogenic responses of cells from diabetic spleens were not due to the induction of suppressor cells. Diabetic rat serum inhibited the proliferation of both control and mitogen-stimulated normal rat lymphocytes. Addition of increasing amounts of VLDL, isolated from diabetic serum, to normal spleen cell cultures also resulted in an exponential decrease in 3H-thymidine incorporation. Complete inhibition of the response of normal spleen cells to PHA was achieved at a VLDL triglyceride concentration of 262 μg/ml. The 4-day-old lym-phyocyte cultures incubated with VLDL at 262-μg/ml concentration were only 11% viable, which was significantly lower than the 58% viability of control cultures without VLDL (P < 0.005). Removal of VLDL from diabetic rat serum eliminated the toxic effect of diabetic serum on spleen cell cultures. Reconstitution of the infra nata n t fraction (d > 1.006) with the VLDL fraction (d < 1.006) restored the toxic activity. These results indicate that the inhibitory effect of diabetic rat serum on the mitogenic responses of normal rat spleen cells is due to the toxicity Of VLDL.

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