Interleukin 2 receptor γ chain expression on resting and activated lymphoid cells

Abstract

The interleukin 2 receptor (IL-2R) is known to be comprised of at least three genetically distinct subunits termed α, β, and γ. These chains can be expressed individually or in various combinations resulting in distinct receptors with different affinities for IL-2. In contrast to α and β, the cell surface expression of the γ chain protein previously has not been well- characterized. To examine cell surface expression of IL-2Rγ on hematopoietic cells, we developed two new monoclonal antibodies (mAbs) specific for this protein. Both 1A11 (immunoglobulin [IgG1]) and 3G11 (IgM) specifically reacted with murine cells transfected with IL-2Rγ cDNA, and immunoprecipitation studies indicated that both antibodies precipitated a protein of approximately 62-65 kD. Scatchard analysis of IL-2 binding to murine cells transfected with cDNA-encoding combinations of IL-2R components demonstrated that neither β nor γ chain bind IL-2 with measurable affinity, but coexpression of both β and γ is sufficient to form an intermediate affinity receptor. In the absence of γ chain, β chain interacts with α chain to form a 'pseudo-high' affinity receptor. In contrast, γ chain does not appear capable of interacting with α chain in the absence of β chain. Thus, γ chain appears to interact only with β, but β chain is capable of interacting with both α and γ. Using the newly developed mAbs to examine cell surface expression by immunofluorescence, resting T cells were found to express low levels of γ chain without detectable α or β. Early after mitogen stimulation, T cells expressed higher levels of α, β, and γ. However, at later time points, T cells expressed α and γ in marked excess over β. Thus, formation of high affinity IL-2R on activated T cells was primarily limited by β chain expression. In contrast, resting natural killer (NK) cells constitutively expressed IL-2Rβ without detectable α or γ. After activation with either IL-2 or IL-12, expression of both α and γ transiently increased and then returned to very low levels. Expression of functional IL-2R on resting and activated NK cells, therefore, appeared to be primarily limited by the expression of γ chain. IL-2 binding studies with resting NK cells confirmed the results of immunofluorescence studies indicating the presence of very low numbers of intermediate affinity (βγ) receptors for IL-2 on these cells. NK cells obtained from patients receiving IL-2 therapy were phenotypically similar to resting NK cells. These studies have identified marked differences in IL-2R composition in two different types of cytotoxic lymphocytes, further underscoring the complexity of this receptor/ligand system. With new reagents specific for IL-2Rγ, it will now be possible to examine further the functional significance of these differences.