Loss of CD28 within CD4+ T cell subsets from cervical cancer patients is accompanied by the acquisition of intracellular perforin, and is further enhanced by NKG2D expression

Abstract

CD28 is well characterized as an essential co-stimulatory receptor critical for activation, proliferation and survival processes in CD4+ T cells. Populations of CD4+CD28null T cells, with apparently contradictory physiological roles, have recently been reported, along with the co-expression of the NK activating receptor NKG2D, in autoimmune diseases and chronic viral inflammation. Paradoxically, studies in cancer suggest that an expanded CD4+NKG2D+ population may be armed with immunosuppressive properties. We have recently reported the existence of two separate CD4+NKG2D+ T cell populations, which were defined by the presence or absence of the co-stimulatory molecule CD28, with the CD4+CD28nullNKG2D+ population more frequently observed in women with cervical cancer. This has led to the present effort to further characterize this population and to determine if the loss of CD28 influences the acquisition of cytotoxic or regulatory markers. In the present work, a multicolor flow cytometry protocol was used to analyze the expression of cytotoxic and immunoregulatory markers on circulating CD4+ T cells characterized by the presence or absence of CD28 and NKG2D in patients with invasive cervical carcinoma and age/gender-matched healthy controls. A noticeable expansion of CD4+CD28null cells, many of them NKG2D+, were observed in selected cervical cancer samples. This CD4+CD28null T cell population was characterized by a lack of immunoregulatory markers, as well as very low basal levels of intracellular IFN-γ, TNF-α, TGF-β, and IL-10. Intracellular perforin, however, was found to be significantly increased in this CD4+CD28null population, and increases in the mean fluorescence intensity of perforin were found to be enhanced by the presence of NKG2D. In conclusion, our data provide the first evidence of a strict link between the absence of CD28 and the expression of perforin, which is likewise enhanced by the expression of NKG2D, within selected CD4+ T cells from cervical cancer patients.