Factors effecting the Th2-like immune response after gamma-irradiation: low production of IL-12 heterodimer in antigen-presenting cells and small expression of the IL-12 receptor in T cells.

Ionizing radiation is known to reduce the helper T (Th) 1-like function, but not the Th2-like function, resulting in a Th1/Th2 imbalance. While this has been known for some time, the mechanism behind the preferential suppression of the Th1 cell activation has not yet been explained. The aim is to elucidate the mechanism in the Th cell imbalance after ionizing irradiation. C57BL/6 mice, 7 weeks old, received whole-body gamma-irradiation (WBI) of 5 Gy. In all instances, the spleen and peritoneal cells were obtained from mice 7 weeks after irradiation. To distinguish Th1 and Th2 cell function, interferon (IFN)-gamma and interleukin (IL)-4 produced by these cells were analysed by an enzyme-linked immunosorbent assay (ELISA). To isolate the primary T cells, the anti-CD90.2 microbead-conjugated antibody was used and the labelled cells were separated by magnetic cell sorting (MACS). To investigate the influence of the IL-12p70 secreted by the antigen-presenting cells, ovalbumin (OVA)-primed peritoneal adherent cells (PAC) were fixed by 1% paraformaldehyde and co-cultured with OVA-specific Th cells in the presence of supernatant of PAC culture with OVA for 16 h. IL-12 receptor, signal transducers and activators of transcription 4 (STAT4) and IFN-gamma expression in the T cells of the WBI mice were detected by reverse transcriptase-polymerase chain reaction (RT-PCR). The spleen lymphocytes of WBI mice showed a depression of IFN-gamma production against OVA, although the total IL-12 was highly secreted. However, the heterodimer IL-12, biologically active protein, was induced less in WBI mice. Although the OVA-specific Th cells were co-cultured with fixed OVA-primed PAC obtained from normal mice, the OVA-specific Th cells showed a decreased IFN-gamma secretion in the presence of the culture supernatant of the activated PAC from the WBI mice. In addition, recombinant IL-12p70 restored the cytokine balance of the OVA-specific Th cells. However the cytokine balance of primary T cells from WBI mice was not completely restored by the normal antigen-presenting cells that abundantly secrete IL-12p70. It was assumed that after WBI, the regenerated T cells also have some problems. It was then observed that the IL-12 receptor expression and intracellular levels of the STAT4 were much lower in the T cells of the WBI mice. The results suggest that the shifted response of the helper T cells after WBI exposure is due not only due to a significant suppression of the secretion of the IL-12p70 in the antigen-presenting cells, but also to the lower expression of the IL-12 receptor on T cells.