Effect of Extended Exposure of Low-dose Radiation on Autoimmune Diseases of Immunologically Depressed MRL/MpJ-gld/gld Mice

We analyzed alterations of splenic T cell subpopulations and amelioration of autoimmune diseases of MRL/MpJ-gld/gld mice (MRL/gld mice) after the extended exposure to low-dose radiation (LDR). Fourmonth old MRL/gld mice were exposed to 0.05, 0.2 and 0.5 Gy/day for 4 weeks (5 days/week) with a total dose of 1, 4 and 10 Gy, respectively. The mice irradiated with 0.2 and 0.5 Gy/day showed an obvious decrease in the proportions of splenic CD4-CD8T cells and remission of their autoimmune diseases. In the mice irradiated with 0.2 Gy/day, apoptotic cells were found in the white pulp of the spleen after the last irradiation, but not in that of the treated MRL/MpJ-+/+ mice (MRL/wild type mice). It seems that the accumulated CD4-CD8T cells are more sensitive to radiation than other T cell subpopulations and prone to apoptosis, and efficient elimination of abnormal CD4-CD8T cells by radiationinduced apoptosis may lead to the amelioration of autoimmune disease. INTRODUCTION James and Makinodan previously reported that whole-body exposure to 0.04 Gy/day of gamma-rays for 20 days changed the proportions of splenic T cell subpopulations to normal levels and improved the conditions of lymphadenopathy and splenomegaly in C57Bl/6J-lpr/lpr mice (C57Bl/lpr mice)(1). The lpr mice are known to have a mutation within the Fas gene (2, 3). Moreover, the autosomal recessive mutation induces autoimmune diseases, nephritis and arthritis, with lymphadenopathy and splenomegaly in approximately five or six-month old mice in the presence of MRL background (4-6). We used MRL/gld mice instead of C57Bl/lpr mice in this study. MRL/gld mice develop a similar phenotype of autoimmune disease as lpr mice with a similar mechanism at about four months of age but have the mutation within a different gene, FasL(7-9). Abnormal CD4-CD8T cells that accumulated due to an insufficient Fas-FasL system induce these autoimmune diseases in lpr and gld mice. In C57Bl/lpr mice, observed by James et al., it was thought that this phenomenon of radiation-induced apoptosis occurred specifically in the abnormal CD4-CD8T cells, and their decrease then participated in the improvement of the conditions of autoimmune diseases. MATERIALS AND METHODS Animals Female MRL/wild type mice as control for genetic background and MRL/gld mice (Japan SLC, Inc., Japan) were used. The mice were given laboratory feed (CE-2, Clea Japan, Japan) and water ad libitum and kept according to the Guiding Principles for the Care and Use of Animals approved in 1987 by the Faculty Meeting of the Univ. of Occup. and Environm. Health, Japan. Irradiation The mice were exposed to gamma-rays from Gammacell 40 Exactor (1.04 Gy/min.) (Nordion Intl. Inc., Canada) or 137Cs simulator (0.0013 Gy/min.)(Sangyoukagaku KK., Japan). Four-month old MRL/gld mice were exposed to 0.05 Gy/day (dose rate; 0.0013 Gy/min.)(Group I), 0.2 Gy/day (dose rate; 1.04 Gy/min.)(Group II) and 0.5 Gy/day (dose rate; 1.04 Gy/min.)(Group III) for 4 weeks (5 days/week) with a total dose of 1 Gy, 4 Gy and 10 Gy, respectively. MRL/wild type mice, four months of age, were exposed to 0.2 Gy/day (dose rate;1.04 Gy/min.)(Group IV) and 0.5 Gy/day (dose rate; 1.04 Gy/min.)(Group V) for 4 weeks (5 days/week). Pathological analysis of autoimmune disease Unirradiated MRL/gld and MRL/wild type mice were sacrificed at four months of age and irradiated MRL/gld mice were sacrificed at four and five months of age. The organs indicated autoimmune diseases, i. e., salivary glands, kidneys and knee joints, were resected and analyzed pathologically. The salivary glands and kidneys were fixed in 10% formalin. The knee joints were treated with 10% formic acid-formalin solution or 14% EDTA (pH 7.4-7.6) solution for decalcification. Organ specimens were stained with hematoxylin and eosin.