Effects of peroxisome proliferators on the thymus and spleen of mice

Abstract

The effects of peroxisome proliferators on the immune system of male C57B1/6 mice have been investigated. Significant atrophy of the thymus and spleen was observed in animals treated with potent peroxisome proliferators (e.g. perfluorooctanoic acid (PFOA), di(2-ethylhexyl)phthalate (DEHP), Wy-14643 and nafenopin), whereas the effects of a moderate peroxisome proliferator (i.e. acetylsalicylic acid (ASA)) were relatively weak. The time course of thymic and splenic atrophy caused by PFOA was found to resemble the time course of the increase in liver weight and of peroxisome proliferation. Analysis of the numbers and phenotypes of thymocytes and splenocytes from PFOA-treated mice revealed the following: (i) the numbers of thymocytes and splenocytes were decreased > 90% and about 50%, respectively, by PFOA treatment; (ii) although all populations of thymocytes were decreased, the immature CD4+CD8+ population was decreased most dramatically; (iii) the numbers of both T and B cells in the spleen were decreased by PFOA treatment. Analysis of the cell cycle of thymocytes indicated that the thymic atrophy caused by PFOA in mice results, at least in part, from inhibition of thymocyte proliferation. Interestingly, in vitro exposure to PFOA for up to 24 h did not produce analogous effects in either thymocytes or splenocytes. Thus, the thymic and splenic atrophy caused by PFOA appears to involve an indirect pathway.

Fig. 1

Time course of the changes in relative liver (•), thymus (▴) and spleen (▪) weights during perfluorooctanoic acid (PFOA) treatment. The results shown are means ±s.d. of four separate experiments (see further Table 2). *P < 0·05; **P < 0·01; ***P < 0·001 compared with the control group.

Fig. 2

Distribution of the DNA content in thymocytes and splenocytes from untreated (a) and perfluorooctanoic acid (PFOA)-treated (b) mice. The cells were stained with propidium iodide (PI). In these frequency histograms: M4, cells with a content of DNA less than that seen in normal cells during the G₀/G₁ phase; M1, cells in the G₀/G₁ phase; M3, cells in the G₂/M phase; M2, this region between the second and the third peaks contains cells in S phase. Experimental conditions were as described in Materials and methods. The results shown are representative of the four independent experiments quantified in Table 4.

Fig. 3

Cell cycle analysis (based on DNA content) of thymocytes (a) and splenocytes (b) isolated from untreated mice and exposed to perfluorooctanoic acid (PFOA) in vitro. The cells were exposed to different concentrations of PFOA for 8 h or 24 h and subsequently stained with propidium iodide. The results shown are means ±s.d. (bars) of four independent experiments.