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. Along similar lines of thought, it is also not clear whether the proteoglycans recently identified in lymphocyte granules are truly representative of these cell types or, as seems more likely, can be found in various immune cell types and therefore may have a much more general function than cytolysis. In general, two strategies used in the last 3 yr have been successful in providing a wealth of novel information on cell-mediated killing. One, involving a reductionist approach taken by several laboratories, attempts to define cytotoxicity in terms of purified polypeptides. This type of biochemical analysis promises to generate an even greater body of information in the next few years. An alternative approach with equally promising perspectives had consisted of studying CTL-specific transcripts. Here, a cDNA library is obtained by subtractive hybridization which supposedly renders it specific for CTL. The transcripts are subjected to further screening by hybridization against messages of different immune cell types, including CTL and non-CTL. CTL-specific transcripts are then sequenced and their function deduced. There is no doubt that this powerful technique will be used more often in the future, especially in the identification of lymphocyte transcripts that are activated by IL-2, lectin and other stimulators. Although we are far from reaching any consensus on how lymphocytes kill their targets, it is emerging from the studies reviewed here that killing mediated by lymphocytes is quite complex, possibly involving multiple mechanisms and mediators that may operate in concert or independently during the cytolytic event. Further biochemical analysis of cell-mediated killing should continue to yield a wealth of information in the years to come.