Office of Biological and Environmental Research

DOE Lowdose Radiation Program Workshop V

2005 Abstract

Title: Mechanisms of Low Dose Radiation-induced T Helper Cell Function

Authors: D.S. Gridley^1,2, L.M. Green¹, J.M. Slater¹ and M.J. Pecaut¹

Institutions: Departments of ¹Radiation Medicine and ²Biochemistry & Microbiology Loma Linda University and Medical Center, Loma Linda, CA 92354 USA

This study investigates the mechanisms that control the balance of T helper (Th) lymphocyte subsets under conditions of low dose, low-linear energy transfer (LET) radiation exposure. The Th cells are essential for generating effective immune defenses against tumor, virus-infected and other aberrant cells.

Our data with C57BL/6 mice indicate that interferon-γ (IFN-γ) production by the Th1 subset is severely compromised following whole-body exposure to 3 Gy radiation in spite of near reconstitution in Th cell number. Studies also show that levels of transforming growth factor-β (TGF-β), a Th3 cell cytokine, are significantly increased under similar conditions. Most recently we have found that low dose radiation (0.05 Gy γ-rays at ~0.03 cGy/hr) enhances potential for IFN-γ production in C57BL/6 mice, based on number of CD3⁺/CD4⁺ Th cells staining positively for intracellular expression of the factor. However, the low dose priming did not result in increased levels of secreted IFN-γ when used alone or when mice were exposed to protracted low dose followed by acute high dose radiation. Collectively, these findings suggest immune deficiency, since IFN-γ plays a key role in both innate and adaptive immune defenses and also has radioprotective, anti-mutagenic and anti-tumor properties. TGF-β is an immunosuppressive, pro-fibrogenic cytokine that allows tumor escape from immune destruction. In addition, reports comparing gene expression profiles after exposure to photons versus protons indicate that differences, as well as similarities, exist. Investigations have not yet focused on genes associated with functions of the various Th cell subsets.

The overall hypothesis of the proposed project is that whole-body exposure to low dose photons (γ-rays) will inhibit Th1 cell function as manifested by insufficient IFN-γ secretion. The Th1 subset deficiency will be accompanied by increased levels of IL-10 and TGF-β1 derived from Th2 and Th3 cells, respectively. In addition, exposure to low dose proton radiation will induce a different pattern of Th subset changes compared to photons. These hypotheses will be addressed in the following Specific Aims: 1) Quantify low dose photon effects on Th lymphocyte gene expression and signal transduction pathways; 2) Determine if Th cell function is significantly altered by low dose photon radiation; 3) Quantify low dose proton effects on Th lymphocyte gene expression and signal transduction pathways; and 4) Determine if Th cell function is significantly altered by low dose proton radiation.

Whole-body low dose irradiation of C57BL/6 mice will be performed throughout this study. In the first aim, animals will be exposed to ⁵⁷Co γ-rays to total doses of 0, 0.01, 0.05 and 0.1 Gy (~ 0.03cGy/hr). Spleens from a subset per group will be evaluated for Th cell gene expression patterns immediately after irradiation (day 0, i.e. within 1-2 hr) using microarrays designed specifically for Th cell biology. In the second aim, spleens from the remaining mice will be assessed for Th1/Th2/Th3 cells based on intracellular cytokines and levels of secreted cytokines on days 0, 4, 20 and 40 post-exposure. In Specific Aims 3 and 4, the same protocols and assays will be performed at the same time points utilizing low dose proton radiation. This will demonstrate whether low-LET radiations, with different track structures, affect Th cells similarly or differently.

Collectively, the data should provide new information on the underlying mechanisms by which low dose, low-LET radiations influence the interactive balance among the Th1, Th2 and Th3 subsets. Depression or dysfunctional interaction among these T cell populations can lead to increased risk for cancer, infectious diseases, allergy and a number of other pathological conditions. The results should also identify productive avenues for countermeasure development.