Studies on the induction and expression of T cell-mediated immunity. VIII. Effector-target junctions and target cell membrane disruption during cytolysis

Intercellular transfer of antigen-presenting cell determinants onto T cells: molecular mechanisms and biological significance

Upon physiological stimulation, receptors with tyrosine kinase activity (RTK) are rapidly internalized together with their soluble ligands. T cell activation is the consequence of recognition by the T cell receptor (TCR) of specific peptide-major histocompatibility protein complexes (peptide-MHC) present at the membrane of antigen-presenting cells (APC). The TCR belongs to the RTK family and is known to be endocytosed upon ligand recognition. It differs from most other RTK in that its ligand, the peptide-MHC complex, is membrane bound and the TCR-ligand interaction is quite weak. Recent experiments have shown that the TCR ligand becomes internalized by T cells upon stimulation. Here we review current knowledge on the molecular mechanisms by which the membrane-bound MHC molecules can be transferred onto T cells, and propose hypotheses on the role this phenomenon could play in physio-pathological situations involving T cells.

Existence of junction-like structures in large B-cell non-Hodgkin's lymphomas. An ultrastructural study

The purpose of this work is to study the presence of cell junction-like structures in large B-cell lymphomas. The ultrastructural study, based on 20 cases of this entity proved by immunohistochemistry and flow cytometry, demonstrated four types of junction-like devices easily found between tumor cells. Several explanations are offered about the possible nature of such structures, including the possibility of them being the result of adhesion phenomena. It should also be emphasized that they can potentially complicate the differential diagnosis of those neoplasms. We should, therefore, be careful in rejecting an ultrastructural diagnosis of large B-cell lymphoma based only on the presence of junction-like structures.

Electron microscopic and immunocytochemical studies of the sequence of events in psoriatic plaque formation following tape-stripping

Electron microscopic and immunocytochemical studies were performed on sequential biopsies following the tape-stripping of the uninvolved skin in 12 patients with psoriasis. In the biopsies taken after 5 min for up to 7 days during the pre-psoriatic phase, there were initial lymphocyte-Langerhans cell interactions as well as interactions between lymphocytes and keratinocytes. In biopsies taken after 6-8 weeks during the proliferative phase there were lymphocyte-macrophage interactions. In the 24-h and 7-day biopsies there were close contacts between epidermal lymphocytes and keratinocytes via microvilli, with blebbing of the keratinocyte plasma membranes and granular cytoplasmic changes around these microvilli. Few basal keratinocyte herniations were noted during this phase. The 6-8-week biopsies of Köbner-positive patients were characterized by a marked increase in lymphocyte-macrophage interactions via similar microvillous processes with associated electronlucent areas suggestive of cytotoxicity.

Efficacy of cyclophosphamide in toxic epidermal necrolysis. Clinical and pathophysiologic aspects

In this article we describe the immunocytochemical and electron microscopic findings in five patients with toxic epidermal necrolysis. They indicate the occurrence of necrotic keratinocytes with nuclear disintegration associated with apposed dendritic cells with the nuclear chromatin configuration of T lymphocytes. These findings, including the presence of blebbing of the keratinocytes and membrane defects associated with cytoplasmic processes from these apposed lymphoid cells, fit known electron microscopic criteria that suggest the involvement of T lymphocyte-mediated cytolysis of drug-altered target keratinocytes in toxic epidermal necrolysis. The effector cell appears to be a dendritic subset, with the phenotypic characteristics (CD3+, CD4-, CD8+, CD2+, DR+) of a T cell subset. There is some evidence that tumor necrosis factor alpha, secreted by activated macrophages, may play a role in necrolysis of the epidermis. The dramatic response of our patients to cyclophosphamide, which is known to inhibit cell-mediated cytotoxicity by inhibiting both the recognition and lethal hit stages, together with the rapid regrowth of the epidermis within 4 days to a week in patients who received adequate dosage of the drug, supports the preceding concepts.

Murine natural killer cell interactions with a fungal target, Cryptococcus neoformans

Earlier investigations have shown that murine natural killer (NK) cells bind to and inhibit the growth of the fungal pathogen Cryptococcus neoformans in vitro and in vivo. To define the stages of NK cell-mediated inhibition of C. neoformans growth and the requirements for the completion of these stages, the events which lead to cryptococcal growth inhibition were compared with those previously elucidated for NK cell-mediated tumor cell lysis. Our data indicate that NK cell-cryptococci binding is a distinct event that precedes inhibition; is temperature independent, although it is slowed at 4 degrees C; and is Mg2+ dependent. In contrast to binding, NK cell-mediated cryptococcal growth inhibition is temperature, Mg2+, and Ca2+ dependent. The removal of Ca2+ by EDTA addition within 3 h after maximal NK cell-cryptococci binding significantly reduced cryptococcal growth inhibition, indicating that Ca2+ is required either late in the NK cell trigger stage or early in the inhibitory stage. These stages and requirements are similar to those previously demonstrated for the model of NK cell-mediated tumor cell lysis; however, the NK cell-cryptococci interactions are somewhat slower than the interactions which culminate in the lysis of the YAC-1 tumor cell targets. These results suggest that C. neoformans cells, although structurally distinct from the standard tumor cell targets, are capable of similar cell-to-cell interactions with NK effector cells as the tumor cell targets.

Perforin-dependent and -independent pathways of cytotoxicity mediated by lymphocytes

There is little doubt at the present time that both perforin-dependent and -independent pathways are important in mediating the cytotoxicity associated with lymphocytes. The cell distribution of perforin, initially thought to include both CTL and NK cells, now must be viewed with caution because all previous biochemical studies on CTL have been conducted with cell lines propagated in long-term cultures in the presence of T cell growth factors (IL-2 and perhaps some still undefined factors). Under these conditions, CTL are known to assume a broader, NK-like specificity in target cell killing and may thus differ significantly from primary CTL generated in the body. Accordingly, perforin does not seem to be present in primary CTL activated directly through mixed lymphocyte reactions. It remains to be shown how primary CTL lyse target cells in vivo. Initial studies conducted in several laboratories have already provided some clues. It now seems that even in cultured, perforin-containing CTL, the perforin pathway is not an obligatory mechanism required for target cell killing. Other pathways, possibly involving TNF/lymphotoxin-like molecules, may play a direct role in this type of cytotoxicity. Other still unidentified factors now also need to be sought, including membrane polypeptides that may develop cytotoxicity directly upon cell contact and binding. Although from the studies reviewed here it is clear now that perforin has a more limited role in cell killing than originally proposed, it is still intriguing that it should share structural and functional homologies with complement proteins, drawing paradoxical analogies between two systems (the cellular and the humoral immune systems) which have evolved to become specialized to carry out separate immunological tasks. The cloning of the genes for perforin and for all the C proteins that comprise the MAC should reveal important information on how these genes originated and then diverged during evolution. The cellular distribution of other granule products, such as serine esterases, also must be viewed with caution. A serine esterase activity was initially thought to be CTL-specific. This information stimulated an intensive research activity in many laboratories that resulted in both the purification of a serine esterase family and the cloning of several serine esterase transcripts. It is becoming clear from recent evidence that this group of enzymes is not truly CTL-specific and therefore would not be expected to develop any function rendered absolutely necessary for cytolysis.(ABSTRACT TRUNCATED AT 400 WORDS)

Study of CTL and LAK contacts to target cells after treatment with mitomycin C and adriamycin

We tried to understand the role of Mitomycin C and Adriamycin in the increased killing of target cells by Cytotoxic T lymphocytes (CTL) and lymphokine activated killers (LAK). For this purpose, we used an objective method allowing quantitative evaluation of the roughness of cell contours on electron micrographs. We compared the folding of the membranes of LAK and CTL as well as conjugated targets exposed to different treatments. We demonstrated first that CTL and LAK displayed similar morphological patterns: the killer cells were more villous than targets in the free areas, and second that the former cells exhibited significant smoothing on the areas of contact with these targets. These results suggest that the binding process (as distinct from the recognition step) is dependent on killer properties which are the same in CTL, LAK and probably NK cells.

Immune cytolysis viewed as a stimulatory process of the target

Humoral and cellular mechanisms of immune cytolysis, as effected by antibody and complement (Ab + C') or by cytolytic T lymphocytes (CTL), have traditionally been considered the end result of early but terminal membrane damage, in turn causing colloid-osmotic lysis of the target cell. A comprehensive theory explaining and relating known prelytic cellular events to subsequent membrane damage is lacking, nor is there a specific picture as to the role and mode of action of Ca2+, which appears to be involved in both complement- and cell-mediated cytolysis (C'MC and CMC, respectively). Recent studies are in support of the view that both Ab + C' and CTL induce a comparable series of prelytic events, in the TC, initiated by membrane depolarization, which in turn bring about voltage-dependent Ca2+ influx or its intracellular release. Persistent elevation of cytosolic Ca2+ can induce massive stimulation of cellular ATPases (actomyosin, Ca2+) and cause exhaustive depletion of ATP. Consequently, Na+-pumping is slowed down and colloid-osmotic lysis ensues. Hence, in our view, membrane damage in immune cytolysis is the result rather than the cause of intracellular events culminating in lysis.

Quantitative ultrastructure of cytolytic lymphocytes mediating allograft rejection in the mouse. I. Cellular alterations in T lymphocytes during specific target cell lysis

A quantitative ultrastructural analysis of cytolytic T lymphocytes (CTL) is presented which allows both the distinction of these cells from normal T lymphocytes and permits the demonstration of ultrastructural alterations of putative CTL following interaction with target cells (TC). Alloreactive CTL were generated in C57BL/10 mice receiving intraperitoneal fibroblastic allografts and target-binding splenic lymphocytes (TBSL) were concentrated by specific immunoadsorption on fibroblast monolayers. TBSL were subjected to ultrastructural quantification either at the onset of TC interaction or following 30 or 60 min incubation at 37 degrees C. By means of simple stereological relationships it was shown that, in comparison with normal, non-cytolytic splenic T lymphocytes, TBSL were slightly larger cells, displaying around 60% more cytoplasm, a similarly-sized nucleus and approximately triple the volume of Golgi apparatus. During the first 30 min of interaction with TC, the target binding surface of the TBSL plasma membrane decreased in area. This change was accompanied by a polarization of the TBSL towards the target. Incubation of lymphocytes with TC for a further 30 min resulted in a general polarization of lymphocytic cellular constituents away from the TC. These results were only attainable by objective quantitative analysis and are discussed in relation to possible mechanisms of CTL-mediated lysis.

Cytotoxic T-lymphocytes. How do they function?

The central theme of this work has been the roles of the CTL receptor and of MHC-proteins in CTL recognition and lysis. A major conclusion that may be deduced from the work presented here is that one CTL receptor is responsible for both target cell recognition and lysis. Although their function as recognitive structures is well established, involvement of MHC-proteins in the events that follow recognition has not been investigated in detail. We have proposed that MHC-proteins are molecular mediators whereby CTL receptors transmit signals ultimately leading to lysis of the target cell. I see future work on CTL-mediated lysis proceeding in the following directions: 1. Verification and analysis of the precise role of MHC proteins in CTL recognition and lysis by use of cell and vesicle systems of defined composition and structure. 2. Study of CTL-mediated 'lethal hit' in systems enabling analysis of early events (millisecond level) preceding lysis. 3. Grafting of CTL receptor(s) activity onto naive cells, using liposomes or other vehicles, and 4. Production of idiotypic reagents such as monoclonal antibodies specific for the combining site/effector mechanism of CTL.

T cell-mediated cytolysis: on the strength of effector-target cell interaction

Allosensitized lymphoid cell populations contain T lymphocytes that can bind to target cells and lyse them. We asked whether there was a relationship between lymphocyte target cell-binding strength and occurrence of cytolysis. Using graded shear forces to dissociate effector-target cell conjugates, we found that (a) within an allosensitized lymphoid cell population derived from an heterogeneous mixed leukocyte culture, there were lymphocyte-target cell conjugates with binding strengths differing by a factor of at least 10(2), (b) even the minimal force required to release a significant amount of bound target cells could disrupt the plasma membranes of some tumor cells and (c) these tumor cells disrupted by shear forces were probably part of cytolysis-conducive rather than of non-cytolysis-conductive conjugates. We combined this approach with the use of cytolysis-inhibiting monoclonal antibodies (mAb), and found that antibody-induced decrease of cytolysis was correlated with a decrease in the percentage of strong or total conjugates, depending on the mAb used. When lectins were added to overcome the inhibitory effect of the mAb, reappearance of cytolytic activity correlated with reappearance of conjugates. This was especially striking using wheat germ agglutinin (WGA): the addition of WGA to irrelevant effector-target cell combinations did not lead to cytolysis; however, the addition of WGA to relevant effector-target cell combinations inhibited by mAb led to reappearance of cytolysis and of strong conjugates. Taken together, these and other results suggested that under our experimental conditions a threshold level of binding strength between effector and target cells might be important, although not sufficient, for T cell-mediated cytotoxicity. These results were not inconsistent with the involvement of mechanical factors in this process. Also, they were in line with the concept of nonantigen-specific lymphoid cell surface interacting molecules, detected by the mAb used and important for the establishment of strong, functional lymphocyte target cell interactions. Finally, they underlined the necessity of a quantitative estimate of cell-cell binding strength when investigating the effect of a given agent (e.g. a mAb) on lymphocyte target cell recognition.

Evidence for cytotoxic T-lymphocyte-target cell interaction in brains of mice infected intracerebrally with lymphocytic choriomeningitis virus

Murine lymphocytic choriomeningitis is a T-cell-mediated pathologic immune phenomenon. The name of this experimental illness was derived from the principal histopathologic alterations of the central nervous system (CNS) of adult mice infected intracerebrally with lymphocytic choriomeningitis virus, i.e., lymphocytic infiltrations of plexus choroidei and meninges. The general assumption that the main event in the pathogenesis is damage to virus-infected target cells by cytotoxic T-lymphocytes is plausible but direct evidence is scarce. We have studied the ultrastructural alterations of both types of cells that are thought to participate in this immunopathologic interaction. Lymphocytes with signs of T-cell transformation were first evident on day 4 after infection. One day later, lymphoblasts, often extending uropods and containing cytoplasmic dense and compound multivesicular bodies, predominated. They were sometimes seen in intimate contact with connective tissue cells of the leptomeninx and epithelial cells of the choroid plexuses which were shown to be infected by immunofluorescence procedure. Lymphoblasts occasionally invaginated the cytoplasm of the putative target cells with cytoplasmic processes, and were even found inside the latter, exhibiting the phenomenon of emperipolesis. Lymphocytic transformation was at its maximum 6 days after infection. At this time, individual leptomeningeal cells and groups of plexus epithelial cells showed signs of cytolysis, and in a few instances these damaged cells were in close spatial association with lymphoblasts. Similar observations have been reported by others who studied the interaction between cytotoxic T-lymphocytes and their appropriate targeted cells in vitro. We interpret our findings as providing direct evidence for the assumption that one link in the chain of events leading to the cerebral form of lymphocytic choriomeningitis of the mouse is damage to virus-infected leptomeningeal and plexus cells by cytotoxic T-lymphocytes.

Human monocyte mediated cytotoxicity to K-562 cells: a freeze-fracture-, scanning- and thin section electron microscopic study

The structural features of "natural"human monocyte-mediated cytotoxicity towards K-562 leukemia cells in vitro have been examined. Human monocytes bound K-562 cells firmly and mediated a slow cytolysis of the leukemia cells. Monocyte binding of target cells was shown to be trypsin sensitive. Freeze-fracture and thin section electron microscopy revealed that effector and target cells were separated by a irregular space larger than 20 nm. There was no evidence for the involvement of specialized membrane junctions or organelle transfer between monocytes and target cells. Scanning electron microscopy revealed that K-562 cells bound to monocytes progressively lost their microvilli. This process started in the membrane areas close to the effector-target cell interaction. The results suggest that binding of target cells by monocytes followed by action of short-range soluble cytotoxic mediators may be the mechanism for the monocyte-mediated cytotoxicity towards K-562 cells.

A fluorescence study on the mobility of surface antigens of untreated tumor cells and of tumor cells undergoing cell-mediated lysis

Heterologous (rabbit) antibodies were raised against murine P-815 mastocytoma cells of DBA/2 origin. Antisera and IgG preparations were highly cytotoxic, whereas Fab fragments thereof lost all activity. Fab fragments also showed a much lower avidity than IgG, both for tumor and normal DBA/2 and C57 spleen cells as measured by the release of iodinated Fab and IgG. Both preparations bound specifically to P-815 cells since they were capable of inhibiting T cell-mediated target cell lysis. The binding of IgG and monovalent Fab fragments was studied by fluorescence. Rhodamine-coupled IgG bound homogeneously in the cold and quickly formed patches upon warming but did not form caps even after prolonged incubation at 37 degrees C. Rhodamine-coupled Fab fragments also bound homogeneously. Their distribution was unaltered after incubation at 37 degrees C even when tumor cells formed uropod-like tails. Fab fragments, however, could be induced to cap with a second and third antibody layer. P-815 cells labeled with rhodamine-coupled Fab fragments were incubated with cytolytic T cells (CTL). The conjugates formed between CTL and fluorescent target cells were observed. No gross redistribution of surface antigens on target cells was observed even at late stages of the lytic process. CTL, therefore, do not seem to operate via a redistribution of surface antigens.