Virus-mediated induction in human lymphocytes of antibody-independent cytotoxicity (VDCC) and enhancement of antibody-dependent cytotoxicity (ADCC) against natural killer-resistant tumor target cells

Biology of natural killer cells

Studies of cytotoxicity by human lymphocytes revealed not only that both allogeneic and syngeneic tumor cells were lysed in a non-MHC-restricted fashion, but also that lymphocytes from normal donors were often cytotoxic. Lymphocytes from any healthy donor, as well as peripheral blood and spleen lymphocytes from several experimental animals, in the absence of known or deliberate sensitization, were found to be spontaneously cytotoxic in vitro for some normal fresh cells, most cultured cell lines, immature hematopoietic cells, and tumor cells. This type of nonadaptive, non-MHC-restricted cellmediated cytotoxicity was defined as “natural” cytotoxicity, and the effector cells mediating natural cytotoxicity were functionally defined as natural killer (NK) cells. The existence of NK cells has prompted a reinterpretation of both the studies of specific cytotoxicity against spontaneous human tumors and the theory of immune surveillance, at least in its most restrictive interpretation. Unlike cytotoxic T cells, NK cells cannot be demonstrated to have clonally distributed specificity, restriction for MHC products at the target cell surface, or immunological memory. NK cells cannot yet be formally assigned to a single lineage based on the definitive identification of a stem cell, a distinct anatomical location of maturation, or unique genotypic rearrangements.

Enhancement of human spontaneous cell-mediated cytotoxicity by a monoclonal antibody against the large sialoglycoprotein (CD 43) on peripheral blood lymphocytes

A murine monoclonal antibody, MoAb B1B6 (IgG1 chi), which recognizes the large sialoglycoprotein (LSGP) on human peripheral blood lymphocytes (PBL) effectively enhanced the spontaneous cytotoxicity of these cells against the natural killer (NK)-sensitive target cells K562 and Molt-4. Whereas preincubation of the lymphocytes with MoAb B1B6 resulted in increased cytotoxicity, preincubation of the target cells had no effect, indicating that the MoAb amplified cytotoxicity at the effector cell level. Kinetic analysis of the data revealed no differences between the control and the MoAb-treated lymphocytes with regard to Vmax, usually considered to reflect the overall lytic potential of the cells. The slopes of the saturation curves, however, differed significantly for the two cell populations, indicating a substantial increment in the activity of the MoAb-treated cells. When studied at the single cell level and with K562 as targets, treatment of PBL with the MoAb resulted in the recruitment of new effector lymphocytes from the pool of non-binding cells. In contrast, when Molt-4 cells were employed as targets, no additional effector cells were recruited. These results indicate that the enhanced cytotoxicity induced by MoAb B1B6 is the result of either recruitment of new effector lymphocytes or of an increased recycling capacity of preexisting effector cells. Together with previous observations, these findings support the conclusion that LSGP belongs to the set of surface molecules which regulate human lymphocyte activation.

Natural cell-mediated cytotoxicity to cells infected with infectious bovine rhinotracheitis virus

Cell-mediated cytotoxicity against viral-infected cells was demonstrated in a 6-hr 51Cr release assay. Peripheral blood mononuclear leukocytes from both infectious bovine rhinotracheitis virus (IBRV)-infected and noninfected cattle exhibited preferential lysis against IBRV-infected primary bovine embryonic kidney (BEK) cells compared to cells infected with pseudorabies virus and noninfected BEK cells. Addition of specific antibody to the assay did not enhance cytotoxicity. The effector cell was a nonadherent cell which was either spontaneously enriched or generated during in vitro cultivation. Maximal cytotoxic activity was detected in peripheral blood mononuclear cells cultured for 3 to 5 days. Several factors affected the magnitude of cytotoxicity during the assay: target cell type, concentration of viral inoculum, duration of effector and target cell contact. It is suggested that target cell lysis was a form of natural cell-mediated cytotoxicity mediated by a cell which has different characteristics from the typical human and murine NK cell.

The role of serologically defined epitopes on mumps virus HN-glycoprotein in the induction of virus-dependent cell-mediated cytotoxicity. Analysis with monoclonal antibodies

The importance of virus structural proteins for the induction of virus-dependent cellular cytotoxicity (VDCC) was studied by means of monoclonal antibodies raised in mice against mumps virions. Antibodies against the viral glycoprotein bearing the haemagglutination and neuraminidase activities (HN) inhibited VDCC but not the natural cytotoxicity (NK) displayed by the lymphocytes in the absence of virus. Antibodies to the fusion factor, the membrane protein or the nucleoprotein were inactive. These results confirmed our previous conclusion, that the only viral component required for VDCC induction is the HN protein. To clarify the role of this protein in VDCC further, the inhibitory activity of 13 HN-specific monoclonals, all of IgG isotype and directed against 9 distinct determinants, was studied in detail. Seven antibodies reacting with 3 different determinants of the peptide moiety of the HN protein were strongly inhibitory. The remaining antibodies, specific for 5 additional peptide epitopes, had intermediate or weak inhibitory effects. One carbohydrate specific anti-HN antibody was inactive although its antigen-binding capacity was of the same magnitude as that of a good inhibitory antibody. The anti-HN antibodies inhibited VDCC regardless of their IgG subclass. Moreover, VDCC inhibition was not correlated with the capacity of the antibodies to inhibit haemagglutination, haemolysis, neuraminidase activity, or the infectivity of the virus. These results suggest that full expression of VDCC requires the interaction of more than one of the serologically defined structures of the HN polypeptide with virus receptors on the lymphocytes and probably also on the target cells. These structures may be different at least in part from those involved in other known biological activities of the virus. Treatment of lymphocytes with virus increases both the number of target-binding cells (TBC) and the number of cytotoxic effector cells. However, when treated under conditions which gave optimal VDCC inhibition, none of the inhibitory antibodies reduced the virus-mediated increase in TBC. This indicates either that the anti-HN antibodies decreased the efficiency of effector-target cell interaction necessary for VDCC induction, or that they blocked a post-binding step required for triggering of cytotoxicity.

Mumps virus-induced enhancement of the in vitro cytotoxicity of cord blood lymphocytes

Purified lymphocytes from the umbilical cord of healthy donors (CBL) displayed lower natural cytotoxicity (NK) and antibody-dependent cellular cytotoxicity (ADCC) than peripheral blood (PBL) from adult donors. In contrast, CBL treated with small amounts of UV-inactivated or live mumps virions expressed the same level of enhanced cytotoxicity (virus-dependent cytotoxicity (VDCC)) against non-infected target cells as PBL. For individual CBL donors there was no correlation between the level of NK and VDCC, indicating involvement of partly distinct effector cell populations. The heterogeneity of the effector cells active in VDCC was confirmed by cell fractionation experiments. The major CBL effector cells in NK and ADCC were found in 'non-T' lymphocyte fractions and/or in fractions containing cells with high-avidity receptors for IgG. In contrast, CBL fractions consisting of about 100% lymphocytes bearing T-cell markers and depleted of Fc gamma R+ cells were strongly cytotoxic in VDCC when T24 cells (human bladder carcinoma) were the targets. With two other target cell types of similar susceptibility to VDCC, the cytotoxic activity of T-cell-containing fractions was less pronounced, indicating that the target cells play an active role in effector cell selection. The surface marker profiles of the VDCC effector cells were the same for CBL and adult PBL. Incubation of CBL with UV-inactivated virions usually gave no significant stimulation of DNA synthesis above that seen in virus-free controls. Taken together, our results suggest that neither specific recognition of viral antigen by T cells nor mitogenic effects of viral material are involved in VDCC generation.