Opsonization of cells by isoantibody in vitro

A Brief Chronicle of Antibody Research and Technological Advances

This review briefly traces the historical development of antibody research and related technologies. The path from early perceptions of immunity to the emergence of modern immunotherapy has been marked by pivotal discoveries and technological advances. Early insights into immunity led to the development of vaccination and serotherapy. The elucidation of antibody structure and function paved the way for monoclonal antibody technology and its application in diagnosis and therapy. Breakthroughs in genetic engineering have enabled the production of humanized antibodies and the advances in Fc engineering, thereby increasing therapeutic efficacy. The discovery of immune checkpoints and cytokines revolutionized the treatment of cancer and autoimmune diseases. The field continues to evolve rapidly with the advent of antibody-drug conjugates, bispecific antibodies, and CAR T-cell therapies. As we face global health challenges, antibody research remains at the forefront of medical innovation and offers promising solutions for the future.

How immunoglobulin G antibodies kill target cells: revisiting an old paradigm

The capacity of immunoglobulin G (IgG) antibodies to eliminate virtually any target cell has resulted in the widespread introduction of cytotoxic antibodies into the clinic in settings of cancer therapy, autoimmunity, and transplantation, for example. More recently, it has become apparent that also the protection from viral infection via IgG antibodies may require cytotoxic effector functions, suggesting that antibody-dependent cellular cytotoxicity (ADCC) directed against malignant or virally infected cells is one of the most essential effector mechanisms triggered by IgG antibodies to protect the host. A detailed understanding of the underlying molecular and cellular pathways is critical, therefore, to make full use of this antibody effector function. Several studies over the last years have provided novel insights into the effector pathways and innate immune effector cells responsible for ADCC reactions. One of the most notable outcomes of many of these reports is that cells of the mononuclear phagocytic system rather than natural killer cells are critical for removal of IgG opsonized target cells in vivo.

Antibody-independent phagocytosis of tumor cells by human monocyte-derived macrophages cultured in recombinant macrophage colony-stimulating factor

Human monocytes exposed in vitro to recombinant macrophage-colony-stimulating factor (rhMCSF) differentiate into monocyte-derived macrophages (MDM), which mediate efficient antibody-dependent cytotoxicity (ADCC) against tumor cells. We and others have shown that this form of ADCC is unusual in that phagocytosis, rather than extracellular lysis, appears to play the major role in target cell killing. In this study, we asked whether the phagocytic form of cytotoxicity seen with ADCC could occur in the absence of an opsonizing antibody. We now report that, whereas cell lines derived from solid tumors are often resistant to antibody-independent cytotoxicity, malignant cells of lymphoid origin appear particularly susceptible to such antibody-independent killing. We found that all of nine lymphocytic leukemia and lymphoma cell lines tested in a total of 35 experiments, plus all four samples of fresh leukemic blasts, were consistently susceptible to antibody-independent MDM cytotoxicity. Antibody-independent cytotoxicity against these cells was efficient (40%-63% killing) at effector: target (E:T) ratios as low as 2:1. Like ADCC, antibody-independent cytotoxicity involved phagocytosis of target cells, as demonstrated by ingestion of fluorescently labeled targets and analysis by flow cytometry. At the time of phagocytosis, the majority of target cells retained membrane integrity, as indicated by the direct transfer of intracellular [51Cr]chromate from radiolabeled targets to phagocytosing MDM, without release of the label into the medium. However, in contrast to ADCC, we found that the degree of antibody-independent cytotoxicity was not a function of the E:T ratio. Instead, a constant proportion of the available target cells were killed regardless of the E:T ratio, suggesting that target cell recognition, rather than effector cell potency, might be the limiting factor in determining cytotoxicity. In additional experiments, we have also identified a second tumor cell type, nueroblastoma, as being susceptible to antibody-independent phagocytosis (all of five cell lines tested, cytotoxicity 40%-93%, E:T = 3:1). Our data thus indicate that the cytotoxicity induced by rhMCSF is not confined to antibody-mediated killing, and that phagocytosis can play a significant role in target cell destruction even in the absence of opsonizing antibody.

Liver macrophage-mediated cytotoxicity toward mastocytoma cells involves phagocytosis of tumor targets

Macrophage-mediated cytotoxicity toward tumor cells usually involves extracellular lysis of the targets. In this study, we report that liver macrophages from rats treated with lipopolysaccharide (5 mg/kg, intravenous) also kill certain tumor cell targets by phagocytosis. Liver macrophages were coincubated with P815 mouse mastocytoma cells for 24 to 72 hr at an effector/target ratio of 10:1. Macrophage phagocytosis was characterized by flow cytometry and by light and electron microscopy. For flow-cytometric studies, P815 cells were prelabeled with the fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate. We found that coincubation of macrophages with labeled targets resulted in a time-dependent increase in macrophage-associated fluorescence, reaching a maximum at 72 hr. This correlated with light-microscopic observations of increased numbers of tumor cells in the macrophages and enhanced macrophage surface area and density. Electron microscopic studies revealed that the initial event in the phagocytic process involved the capture of P815 cells by the pseudopodia of the macrophages. Target cells were then surrounded by lamellipodia, internalized in phagosomes and destroyed. These data, together with previous studies, provide evidence for multiple mechanisms of cytotoxicity mediated by activated liver macrophages.

Macrophages induce antibody-dependent cytostasis but not lysis in guinea pig leukaemic cells

Guinea pig and mouse peritoneal macrophages formed antibody-dependent rosettes with guinea pig L2C leukaemic cells, but were unable either to phagocytose the cells or to kill them extracellularly as judged by the retention of 51Cr. Macrophages previously activated by BCG in vivo also failed to exhibit phagocytosis or cytoxicity towards the antibody-coated cells. These failures could not be attributed to deficient function of the macrophages nor to antigenic modulation of the L2C cells. The antibodies involved were capable of mediating lysis by complement, and ADCC by human leukocytes. However macrophages were cytostatic to antibody-coated L2C cells in that uptake of 3H-thymidine or 3H-deoxycytidine was abruptly and in some cases completely inhibited upon cell contact being established. Antigenic modulation which had proceeded sufficiently to protect against lysis by complement did not protect against cytostasis. Syngeneic macrophages had greater cytostatic activity than did allogeneic or xenogeneic. Macrophage activation by BCG did not result in significantly increased cytostasis. A univalent antibody derivative Fab/c was also capable of mediating cytostatis by the macrophages.

Murine natural anti-tumor antibodies. II. The contribution of natural antibodies to tumor surveillance

The objective of this study was to investigate whether natural anti-tumor antibodies (NAb) contributed to the surveillance of small inocula of syngeneic tumors. Experiments were designed to distinguish between NAb and NK-mediated host resistance. Four approaches were used: (1) the isolation of tumor variants differing in their tumorigenicity and susceptibility to these mechanisms, and to activated macrophages; (2) a comparison of the effects of adjuvants on the modification of both host resistance and the activity of these effectors; (3) the relationship between the ontogeny of the natural resistance mechanisms and tumorigenicity in aged mice; and (4) the use of a Winn-type assay. These studies provided support for a role in natural resistance correlated with both the ability of tumors to bind NAb, and the production of NAb in adjuvant-stimulated mice. Furthermore, the frequency of tumors observed after tumor challenge more closely correlated with the ontogeny of natural antibody than NK cells, and tumors coated with NAb were less tumorigenic than controls. The reduced tumorigenicity of an NK-sensitive tumor, when compared to an NK-resistant variant of the same line, provided evidence for NK-cell-mediated natural resistance in young adult mice. It was concluded that natural resistance to tumors is a complex phenomenon dependent on the tumor phenotype, as well as the activity of several effector mechanisms, and that natural anti-tumor antibody must be considered an important component of host resistance.

Antibody-mediated suppression of lymphoma: participation of platelets, lymphocytes, and nonphagocytic macrophages

Humoral antibody directed against a grafted murine lymphoma can suppress the growth of the tumor in mice of the inbred strain native to the tumor. Antibody, however, cannot suppress the tumor growth in mice given 500 R of whole-body irradiation. When the tumor-antibody inoculum is admixed with lymphocytes or macrophages obtained from peritoneal exudate, macrophages experimentally rendered nonphagocytic, or with platelets isolated from peripheral blood, the tumor growth is suppressed in irradiated mice. These results indicate that mechanisms other than phagocytosis may play an important role in antibody-mediated suppression of tumor growth in vivo.