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Although T cell receptors and Fc receptors are the best known cytotoxic triggering molecules, a number of adhesion molecules recently have been identified as alternative triggering molecules. We discuss how CD44, an adhesion molecule found on all leukocytes and many other cell types, becomes a triggering molecule on NK cells following stimulation with IL-2. We also describe a genetically engineered single chain bispecific antibody, produced in mammalian cells and in bacteria, that is capable of redirecting lysis in the nanogram per milliliter range.

T cells can be induced to specifically lyse tumor cells with bispecific antibodies containing anti-T cell receptor mAbs crosslinked to anti-tumor mAbs. Such "targeted cytolysis" requires that the target cell be bound directly to the cytotoxic cell. In addition, targeted T cells mediate a second activity, the secretion of factors that can block the growth of both tumor target cells and bystander tumor cells. When given to nude mice bearing intraperitoneal human ovarian carcinoma, targeted human T cells cause the rapid removal of most tumor cells from the peritoneum, and markedly prolong the times of survival of treated mice. The efficacy of targeted T cells for treating human cancer is currently being tested in clinical trials.

It has been well established that bispecific antibodies containing anti-T-cell receptor MAbs crosslinked to anti-tumor MAbs induce T cells to lyse tumor cells, as measured in a 51Cr-release assay. Such lysis requires direct attachment between target and cytotoxic cells and most probably involves the exocytosis of cytolytic substances into the cell:cell interface. In addition, targeted T cells mediate a second activity, the secretion into the medium of factors that can block the growth of bound tumor cells and unbound bystander cells. In order to test how targeted effector cells mediate anti-tumor effects in vivo, we are currently developing a totally syngeneic murine system in which murine T cells are targeted against mouse mammary tumors. The system allows us to treat both primary tumors and tumor transplants, using a mammary-tumor-virus antigen as the entity that is specifically recognized on the tumor cells.

Immunoelectron microscopical studies performed in healthy human neutrophils showed the presence of glycosyl-phosphatidylinositol (GPI)-linked CD67 in granules. The use of immunogold double-labeling of CD67 and lactoferrin (LF; as marker for specific granules) or CD67 and myeloperoxidase (MPO; as marker for azurophilic granules) showed that CD67 occurred only in the specific granules. Furthermore, flow cytometry showed that CD67 has a low level of expression on the plasma membrane of these cells. In paroxsymal nocturnal hemoglobinuria (PNH)-affected neutrophils, CD67 was not detected in any intracellular compartment by immunoelectron microscopy, and flow cytometry showed no CD67 on the plasma membrane. In earlier studies, FcRIII was found on the plasma membrane, in electron-lucent vesicles, and in the Golgi complex of healthy neutrophils, and in the Golgi complex of some of the PNH-affected neutrophils. Here we have studied FcRIII in PNH-affected cells of three other patients and found, by immunoelectron microscopy, that the receptor can not be detected in these cells. However, flow cytometry showed that FcRIII was not completely absent on the plasma membrane of the affected cells, but that the level of expression on these cells was low. Thus, PNH patients can differ from one another with respect to the occurrence of affected neutrophils that have a detectable level of FcRIII in the Golgi complex. In summary, these findings show not only that the expression of the two GPI-linked proteins, CD67 and FcRIII, is markedly lower on the plasma membrane, but also that neither occurred in any of the intracellular compartments of affected neutrophils of the PNH patients examined in this study.

The internalization of peroxidase-antiperoxidase (PAP) immune complexes by human neutrophil granulocytes was studied at an ultrastructural level. PAP initially bound to the plasma membrane at 4 degrees C accumulated in endosomes within 5 min of internalization. By that time, all of the ligand bound to the plasma membrane had been removed from the cell surface and the cells were able to bind newly added PAP. Pre-embedding labelling of FcRIII on these cells showed that this receptor was present on the cell surface, indicating involvement of FcRIII in the rebinding of PAP in neutrophils. The origin of FcRIII present on the plasma membrane after Fc receptor-mediated internalization of PAP was investigated in another series of experiments. Incubation of the cells with pronase eliminated the epitope on the Fc receptor recognized by anti-FcRIII. After the pronase treatment hardly any Fc receptors were detected on the plasma membrane. However, incubation of the cells for only 5 min in a protease-free medium after the pronase treatment led to an abundance of FcRIII on the plasma membrane of the neutrophils. These findings support the hypothesis that FcRIII on the plasma membrane of human neutrophil granulocytes is replenished from an internal source of free Fc receptors and suggest that at least some of the receptors present on the cell surface after the binding and internalization of PAP originate from this source in the cytoplasm.

Human blood neutrophils bear two types of Fc receptors that recognize the Fc portion of immunoglobulin G: FcRII and FcRIII. In earlier studies we found that neutrophils not only express FcRIII on their plasma membrane but also contain a large population of FcRIII-containing vesicles mainly located in the juxtanuclear area. To find out whether these vesicles derive from the plasma membrane, we used electron microscopic techniques to study compartments involved in ligand-independent endocytosis in human neutrophil granulocytes. The endocytic compartments were labeled with BSA-gold. This marker entered the cell through non-coated invaginations of the plasma membrane as well as via coated pits. After internalization, BSA-gold was present in numerous electron-lucent vesicles in the juxtanuclear area and in the trans-Golgi reticulum, endosomes, and lysosome-like structures. FcRIII also occurred in the BSA-gold-containing electron-lucent vesicles in the juxtanuclear area, as shown by postembedding immunocytochemical labeling of FcRIII in cells already containing BSA-gold. Quantification showed that 29% of all FcRIII-containing vesicles also bear BSA-gold while the remaining 71% contain only the receptor. In sum, our findings show that one third of the FcRIII-containing electron-lucent vesicles in neutrophil granulocytes derive from the plasma membrane and are involved in ligand-independent endocytosis of FcRIII. The majority of these vesicles, however, are not of an endocytic origin and might constitute an "internal pool" of receptors in these cells.

Immunoelectron microscopic studies in human neutrophils showed that FcRIII was present on the plasma membrane, in the Golgi complex, and in many small vesicles (120 to 180 nm). FcRIII was not found in specific or azurophilic granules as shown by immunogold double-labeling experiments, visualizing both FcRIII and either lactoferrin (a marker of specific granules) or myeloperoxidase (a marker for azurophilic granules). Because the occurrence of FcRIII in the Golgi complex suggested that biosynthesis of this receptor occurs in these cells, metabolic labeling experiments were performed. Immunoprecipitation of FcRIII from NP-40 lysates of cells labeled with 35S-methionine showed a diffuse 50- to 70-Kd band corresponding with the band noted after immunoprecipitation of FcRIII from surface iodinated cells. These findings show that de novo synthesis of FcRIII occurs in neutrophils and suggest that at least some of the small vesicles containing FcRIII derive from the Golgi complex and thus are involved in transport of newly synthesized FcRIII to the plasma membrane.