Apoptotic pathways are selectively activated by granzyme A and/or granzyme B in CTL-mediated target cell lysis

Purified cytolytic T lymphocyte (CTL) proteases granzyme (gzm)A and gzmB with sublytic dose of perforin (perf) initiate distinct proapoptotic pathways. Their physiological relevance in CTL-mediated target cell apoptosis is elusive. Using ex vivo virus-immune CD8+ T cells from mice deficient in perf, gzmA and/or gzmB, and the Fas-resistant EL4.F15 tumor target cell, we show that (a) CTL from gzmA−/− or gzmB−/− mice similarly induced early proapoptotic features, such as phosphatidyl serine (PS) exposure on plasma membrane, ΔΨm loss, and reactive oxygen radical generation, though with distinct kinetics; (b) CTL from gzmA−/− but not from gzmB−/− mice activate caspase 3 and 9; (c) PS exposure induced by CTL from gzmA−/− or gzmB−/− mice is prevented, respectively, by caspase inhibitors or by reactive oxygen scavengers without interfering with target cell death; and (d) all gzm-induced apoptotic features analyzed depend critically on perf. Thus, perf is the principal regulator in CTL-mediated and gzm-facilitated intracellular processes. The ability of gzmA and gzmB to induce multiple independent cell death pathways may be the hosts response to circumvent evasion strategies of pathogens and tumors.

Loss of tumor-specific and idiotype-specific immunity with age.

The ultraviolet light-induced fibrosarcoma 1591 undergoes "first-set rejection" when transplanted into normal syngeneic mice. We found, however, that the primary resistance of normal mice decreases with age, beginning at 9--12 mo, equivalent to middle age for mice. Mice lose with age the capacity to mount both idiotypic and anti-idiotypic responses responsible for controlling the growth of tumor. This loss was correlated with quantitative as well as qualitative changes in the response, such as changes in specificity and clonotype. Normal young mice regularly expressed a dominant common anti-1591 "idiotype" as defined by an anti-idiotypic probe. The capability of normal mice to respond with lymphocytes of this dominant common idiotype began to decline at about 8 mo of age. At this time, animals still generated tumor-specific lymphocytes, but these lymphocytes appear to be idiotypically different lymphocyte clones. With further increase in age, animals responded with tumor-reactive lymphocytes that showed a marked cross-reactivity to other tumor target cell lines. Both in vivo and in vitro, the capability of normal mice to mount an immune response that was specific for the 1591 tumor cells decreased between 9 and 14 mo, which was the age individual mice became increasingly susceptible to a challenge with 1591 tumor cells. Thus, our data suggest that clones of tumor-specific T cells provide primary and early protection of young animals against challenge with malignant 1591 cells. However, the dominance of these tumor-specific T cell clones in a primary immune response is lost in middle-age. Because the ability of animals to mount anti-idiotypic immune response also declined in middle-aged animals, it is possible that the observed loss of clonal dominance of tumor-specific clones with increasing age is at least partially related to age-dependent changes in the anti-idiotypic compartment.