Regulation of interferon-gamma production in human peripheral blood mononuclear cells: Heredity, relation to markers of allergy and possible role of inducible T cell kinase (ITK)

Abstract

IFN-γ and IL-4 are cytokines that are thought to be products of distinct differentiation pathways for lymphocytes and that inhibit and enhance the development of IgE antibodies, respectively. We hypothesized that the level of IFN-γ production upon stimulation of immune cells in the blood is a stable, heritable phenotype that is inversely related to phenotypic markers of allergy and to the Th2 cytokine IL-4. The studies were performed with samples of human blood obtained from children enrolled at birth in a large, longitudinal study of allergy known as the Tucson CRS. Mitogen-stimulated IFN-γ production by PBMCs of children at age 11 was compared to IFN-γ production in the first year of life and found to show a significant positive correlation. The expected inverse relation to markers of allergy (total serum IgE, skin test reactivity and eosinophils) was not observed. A strong positive relation was observed between IFN-γ production in the parents and children, lending support to the concept of genetic regulation. In contrast to the inverse relation hypothesized to IL-4, a positive relation was observed to IL-4 as well as to another cytokine, IL-2. A linkage analysis performed by a colleague showed linkage between low production of IFN-γ and a region of chromosome 5q in which the gene for a T-cell specific kinase (Itk) is located. These results led us to hypothesize a common "regulator" of cytokines that was functioning and might be Itk. Decreased expression and/or decreased function of Itk could account for the low cytokine data. In order to test this hypothesis, ten adult volunteers with low and ten with high cytokine production matched for age and sex were tested for Itk function and expression. The two groups were examined for Itk expression (normalized to CD3 expression) by western blotting. The group of low cytokine producing individuals showed significantly less Itk expression than the high cytokine-producing group. Attempts to optimize an in vitro kinase assay to determine Itk function were unsuccessful. In the absence of functional studies to support the results from the expression studies, it is not possible to say with certainty that Itk function is related to cytokine production. Therefore, we conclude that IFN-γ expression in humans is a stable phenotype with evidence of a genetic basis for regulation and that Itk is a candidate gene contributing to that genetic basis.