Murine strain variation in the natural killer cell and proliferative responses to the immunostimulatory compound 7-allyl-8-oxoguanosine: role of cytokines.

7-Allyl-8-oxoguanosine (loxoribine) is a di-substituted guanine ribonucleoside which has been shown previously to enhance murine NK activity, B lymphocyte proliferation, and antibody synthesis. In this study we examined the relationship among enhancement of NK activity, proliferation, and cytokine synthesis in the responses of different strains of mice to loxoribine to provide insight into the role of cytokines in these biological activities. The NK response of mice was enhanced both in vitro and in vivo in all strains tested with the exception of the NK-deficient beige (BgBg) mouse. However, there was a marked difference in the degree of NK enhancement noted in other inbred strains, with C3H and CBA mice producing the highest responses, C57BL/6, BALB/c, and DBA/2 strains giving intermediate responses, and SJL mice manifesting low responses. Striking enhancement of NK cell activity was seen in SCID mice. A somewhat different effect was seen in humans. Loxoribine treatment enhanced both the NK and LAK activity of cells from individuals with low and high spontaneous NK activity. The degree of enhancement was similar for both groups, and thus the general hierarchy of NK activity among different donors was maintained. There was less interstrain variation in the murine proliferative response to loxoribine although nude (NuNu) mice showed the highest activity and SJL mice produced substantially lower responses than other strains. All strains produced IL-6, TNF alpha, IFN-alpha/beta, and IFN-gamma when spleen cells were cultured for 48 hr with loxoribine. Interstrain variability of cytokine synthesis displayed no consistent pattern from one cytokine to another, and all failed to correlate with interstrain variability of NK cell activity or B lymphocyte proliferation. When anti-cytokine antibodies were tested for the ability to block the immunostimulatory effects of loxoribine, only anti-IFN-alpha/beta and, to a lesser degree, anti-IFN-beta, partially inhibited NK activation. Similarly, only anti-IFN alpha/beta antibodies partially blocked the proliferative response to loxoribine. In both cases, reconstitution of the responses was achieved by adding back IFN-alpha/beta to cultures containing antibodies against IFN-alpha/beta. These data suggest that, although several cytokines are produced in response to loxoribine, only IFN-alpha and IFN-beta are directly involved in the NK activation and proliferative responses. The pattern of strain variation appears to be reflective of variation in NK cell responsiveness to IFN-alpha/beta.