High numbers of large granular lymphocytes (LGL) accumulate in the livers and peritoneal cavities of mice during the course of viral infection. Accumulation of natural killer (NK) cells at day 3 postinfection (p.i.) was shown to be radiation-sensitive, implying that proliferation was required for this response. Accumulation occurred in splenectomized mice, indicating that the spleen, known to be an organ for mature NK cell proliferation, was not the major source for liver and peritoneal NK/LGL. Significant percentages (greater than 25%) of the LGL found in the liver and peritoneal cavity following viral infection or interferon induction with poly-inosinic:poly-cytidylic acid were defined morphologically as blasts (large cells with prominent nucleoli and intensely basophilic cytoplasms containing azurophilic granules). Most blast LGL at day 3 p.i. were sensitive to administration of anti-asialo GM1 serum in vivo, were Lyt-2-, and were enriched in populations that lysed NK cell-sensitive targets in vitro, indicating that these were NK/LGL. At day 3 p.i., leukocytes from the liver and peritoneal cavity incorporated 3H-thymidine and bound to and killed NK cell-sensitive targets in single-cell cytotoxicity assays. These data suggest that NK/LGL undergo at least one round of division in the liver and peritoneal cavity during viral infection. In contrast, blast LGL at day 7 p.i. were resistant to in vivo treatments with anti-asialo GM1 serum, were Lyt-2+, and were enriched in populations of cells that killed virus-infected histocompatible targets, indicating that they were cytotoxic T lymphocytes (CTL). These results suggest that both NK/LGL and CTL/LGL are capable of blastogenesis and presumed proliferation at sites of virus infection, providing a means for the in situ augmentation of a host's cell-mediated antiviral defenses.

To assess the effects of chronic virus infection on NK cells, the related phenomena of interferon (IFN) production, NK cell activation, and resistance to tumor implants were studied in mice persistently infected with lymphocytic choriomeningitis virus (LCMV). NK cells from these LCMV-carrier mice displayed augmented killing of the NK-sensitive YAC-1 target cell. They did not lyse the more resistant targets L-929 and P815, whereas NK cells from acutely infected mice efficiently lysed all three cell types. The plasma from LCMV-carrier mice contained an antiviral substance identified as IFN type I, based on species specificity, virus nonspecificity, resistance to pH 2, and sensitivity to antibody to type I IFN. IFN titers in plasma from LCMV-carrier mice were 32 to 64 U/ml, about 20-fold less than those in acutely infected mice. Both the IFN and NK cell levels continuously remained elevated in the LCMV carrier mice up to at least 6 months of age. IFN is known to activate NK cells and to induce their blastogenesis in vivo. As determined by centrifugal elutriation, large NK blast-size cells were isolated from the spleens of acutely infected mice, but not from either normal or LCMV-carrier mice, suggesting augmented NK cell-mediated lysis in the absence of enhanced proliferation. Poly inosinic-cytidylic acid induced high levels of NK cell-mediated cytotoxicity and blastogenesis in both control and LCMV-carrier mice, but IFN was induced to lower levels in carriers as compared with controls. Coincidental with augmented NK cell activity, the LCMV-carrier mice rejected intravenously injected 125IUdR-labeled tumor cells more efficiently than did normal mice. Thus, LCMV carrier mice have low levels of type I IFN, moderately augmented NK cell activity lasting for at least 6 months, and increased resistance to tumor cell implants. This indicates that augmented NK cell-mediated cytotoxicity can be maintained in vivo over prolonged periods of time in the presence of chronic low-level IFN stimulation.