Gideon Berke

The CTL's kiss of death

The potent and specific lytic activity of CTLs can occur by at least two distinct pathways. In the secretion and perforin-mediated pathway, the direct effect(s) on the target cell membrane of the pore-forming agent perforin, probably in conjunction with granzymes, also secreted from the CTLs, causes the targets demise. Intercytoplasmic transfer of granzymes is believed to be involved in inducing target apoptosis. In the Fas-mediated pathway, engagement of a CTL membrane ligand with an apoptosis-inducing target cell surface receptor, such as the FasL with Fas, triggers programmed disintegration of the CTL-bound target; secretion of granzymes and pore formation by perforin are not involved in this receptor-mediated mechanism. Despite the fundamental differences in their onset for both pathways, the downstream sequence of events that culminate in target cell apoptosis appears to be similar. Further studies will resolve this enigma.

Runx3 and Runx1 are required for CD8 T cell development during thymopoiesis

The RUNX transcription factors are important regulators of lineage-specific gene expression. RUNX are bifunctional, acting both as activators and repressors of tissue-specific target genes. Recently, we have demonstrated that Runx3 is a neurogenic transcription factor, which regulates development and survival of proprioceptive neurons in dorsal root ganglia. Here we report that Runx3 and Runx1 are highly expressed in thymic medulla and cortex, respectively, and function in development of CD8 T cells during thymopoiesis. Runx3-deficient (Runx3 KO) mice display abnormalities in CD4 expression during lineage decisions and impairment of CD8 T cell maturation in the thymus. A large proportion of Runx3 KO peripheral CD8 T cells also expressed CD4, and in contrast to wild-type, their proliferation ability was largely reduced. In addition, the in vitro cytotoxic activity of alloimmunized peritoneal exudate lymphocytes was significantly lower in Runx3 KO compared with WT mice. In a compound mutant mouse, null for Runx3 and heterozygous for Runx1 (Runx3-/-;Runx1+/-), all peripheral CD8 T cells also expressed CD4, resulting in a complete lack of single-positive CD8+ T cells in the spleen. The results provide information on the role of Runx3 and Runx1 in thymopoiesis and suggest that both act as transcriptional repressors of CD4 expression during T cell lineage decisions.

Regression of established murine carcinoma metastases following vaccination with tumour-associated antigen peptides

The cure of micrometastases following surgery is the major goal of cancer immunotherapy. We have recently isolated tumour-associated antigen (TAA) peptides, MUT 1 and MUT 2, derived from a mutated connexin 37 gap-junction protein, from the malignant 3LL-D122 murine lung carcinoma. We now report that synthetic MUT 1 or MUT 2 induces effective antitumour cytoxic T lymphocytes. Peptide vaccines protect mice from spontaneous metastases of 3LL-D122 tumours. Moreover, peptide vaccines reduce metastatic loads in mice carrying pre-established micrometastases. Tumour-specific immunity was primarily mediated by CD8+ T cells. This is the first evidence that peptide therapy may be effective in treatment of residual tumours and provides a rationale for the development of peptide vaccines as a modality for cancer therapy.

Fas (CD95/Apo-1)–Mediated Damage to Ventricular Myocytes Induced by Cytotoxic T Lymphocytes From Perforin-Deficient Mice: A Major Role for Inositol 1,4,5-Trisphosphate

Cytotoxic T lymphocytes (CTLs) that infiltrate the heart are important immune effectors implicated in heart transplant rejection, myocarditis, and other cardiomyopathies. To investigate the mechanism(s) underlying CTL damage to the myocardium through activation of the Fas receptor (Fas/CD95/Apo-1) by the Fas ligand, we explored the interaction between peritoneal exudate CTLs (PELs), derived from perforin gene-knockout (P-/-) mice, and murine ventricular myocytes. Fas expression on isolated ventricular myocytes was demonstrated immunohistochemically. Action potentials, [Ca2+]i transients, and contractions of myocytes conjugated to P-/- PELs or treated with the apoptosis-inducing anti-Fas monoclonal antibody Jo2 were recorded. Action potential characteristics of nonconjugated myocytes and myocytes conjugated with P-/- PELs were, respectively, as follows: Vm, -73.2+/-1.5 and -53.6+/-6.4 mV (mean+/-SEM); action potential amplitude, 117.9+/-3.9 and 74.3+/-21.2 mV; and action potential duration at 80% repolarization, 17+/-6 and 42+/-13 milliseconds (all P<.05). P-/- PELs also induced early and delayed afterdepolarizations as well as arrhythmogenic activity. Diastolic [Ca2+]i increased during the cytocidal interaction with P-/- PELs, from a fluorescence ratio of 0.82+/-0.05 (n=7) to 1.98+/-0.09 (n=13) (P<.05). All of the effects caused by P-/- PELs were reproduced by incubating the myocytes with Jo2. Heparin (50 microg/mL), an antagonist of inositol trisphosphate (IP3)-operated sarcoplasmic reticulum Ca2+ channels, or U-73122 (2 micromol/L), a phospholipase C inhibitor, but not the inactive agonist U-73343, prevented Fas-mediated myocyte dysfunction. Additionally, intracellular application (through the patch pipette) of the active IP3 analogue, inositol 1,4,5-trisphosphate, but not the inactive analogue, inositol 1,3,4-trisphosphate, caused electrophysiological changes resembling those resulting from P-/- PELs and Jo2, suggesting that CTL-induced Fas-based myocyte dysfunction is mediated by IP3. We conclude that a Fas-based perforin-independent mechanism of CTL action can account for the immunopathology seen in the allotransplanted heart, myocarditis, and dilated cardiomyopathy.

CD40 Ligation Activates Murine Macrophages via an IFN-γ-Dependent Mechanism Resulting in Tumor Cell Destruction In Vitro

We have shown previously that agonistic anti-CD40 mAb induced T cell-independent antitumor effects in vivo. In this study, we investigated mechanisms of macrophage activation with anti-CD40 mAb treatment, assessed by the antitumor action of macrophages in vitro. Intraperitoneal injection of anti-CD40 mAb into C57BL/6 mice resulted in activation of peritoneal macrophages capable of suppressing B16 melanoma cell proliferation in vitro, an effect that was greatly enhanced by LPS and observed against several murine and human tumor cell lines. Anti-CD40 mAb also primed macrophages in vitro to mediate cytostatic effects in the presence of LPS. The tumoristatic effect of CD40 ligation-activated macrophages was associated with apoptosis and killing of tumor cells. Activation of macrophages by anti-CD40 mAb required endogenous IFN-γ because priming of macrophages by anti-CD40 mAb was abrogated in the presence of anti-IFN-γ mAb, as well as in IFN-γ-knockout mice. Macrophages obtained either from C57BL/6 mice depleted of T and NK cells by Ab treatment, or from scid/beige mice, were still activated by anti-CD40 mAb to mediate cytostatic activity. These results argued against the role of NK and T cells as the sole source of exogenous IFN-γ for macrophage activation and suggested that anti-CD40 mAb-activated macrophages could produce IFN-γ. We confirmed this hypothesis by detecting intracytoplasmic IFN-γ in macrophages activated with anti-CD40 mAb in vivo or in vitro. IFN-γ production by macrophages was dependent on IL-12. Taken together, the results show that murine macrophages are activated directly by anti-CD40 mAb to secrete IFN-γ and mediate tumor cell destruction.

Synergistic Activation of Macrophages via CD40 and TLR9 Results in T Cell Independent Antitumor Effects

We have previously shown that macrophages (Mφ) can be activated by CD40 ligation to become cytotoxic against tumor cells in vitro. Here we show that treatment of mice with agonistic anti-CD40 mAb (anti-CD40) induced up-regulation of intracellular TLR9 in Mφ and primed them to respond to CpG-containing oligodeoxynucleotides (CpG), resulting in synergistic activation. The synergy between anti-CD40 and CpG was evidenced by increased production of IFN-γ, IL-12, TNF-α, and NO by Mφ, as well as by augmented apoptogenic effects of Mφ against tumor cells in vitro. The activation of cytotoxic Mφ after anti-CD40 plus CpG treatment was dependent on IFN-γ but not TNF-α or NO, and did not require T cells and NK cells. Anti-CD40 and CpG also synergized in vivo in retardation of tumor growth in both immunocompetent and immunodeficient mice. Inactivation of Mφ in SCID/beige mice by silica treatment abrogated the antitumor effect. Taken together, our results show that Mφ can be activated via CD40/TLR9 ligation to kill tumor cells in vitro and inhibit tumor growth in vivo even in immunocompromised tumor-bearing hosts, indicating that this Mφ-based immunotherapeutic strategy may be appropriate for clinical testing.

T-lymphocyte-mediated cytolysis as an excitatory process of the target: I. Evidence that the target cell may be the site of Ca2+ action

Delivery of the lethal hit signal to target cells (TC) by cytolytic T lymphocyte (CTL) has traditionally been considered strictly dependent upon the presence of external Ca2+ [( Ca2+]ext) in the medium, but neither the role of Ca2+ nor its site of action (effector or target) have been known. We have observed that in different CTL/TC systems the requirement for [Ca2+]ext varies, depending on the target. Some TC, like leukemia L1210, are strictly dependent on [Ca2+]ext for lysis while others, like EL4 (and P815), are not. It is therefore suggested that, where required, [Ca2+]ext exerts its effect(s) on the TC and not the CTL. In support of this conclusion are experiments showing that effector cells cytolytic to certain TC in the absence of [Ca2+]ext, require [Ca2+]ext when used themselves as TC of other effectors. Verapamil, a Ca2+-channel blocker, inhibits the lysis of L1210 but not of EL4 cells, suggesting involvement of Ca2+ flux into L1210 target cells and, if at all involved, Ca2+ mobilization from internal stores in EL4. The different lytic susceptibility of the two TC to the Ca2+ ionophore A23187, in the presence and absence of [Ca2+]ext, correlated with their responses to CTL. It suggests Ca2+ influx into both types of TC in the presence of [Ca2+]ext and its release from internal stores in the lysis of EL4 but not L1210 in the absence of [Ca2+]ext. In view of these results indicating that the target is the site of Ca2+ action, we propose that CTL induce a Ca2+-regulated activation of the TC leading to its lysis.

Unlocking the secrets of CTL and NK cells

Elucidation of the precise molecular mechanism(s) by which cytotoxic T lymphocyte (CTL) and natural killer (NK) cells recognize and kill their targets continues to be the subject of intense research. During the 198Os, a mechanism for lysis mediated by CTL/NK cells was proposed that entailed the formation of pores in the membranes of target cells by secreted molecules of CTL/NK origin, such as the pore-forming protein perforin and a series of unique granule serine proteinases (granzymes) (Fig. la, right). In parallel, the existence of an alternative and nonsecretory lytic mechanism, which does not utilize perforin and granzymes (Fig. la, left), was indicated by demonstrations that: (1) CTLs can lyse certain target cells effectively in Ca 2+-free medium, a condition in which perforin is neither secreted nor cytolytic; and (2) some potent CTLs, such as alloimmunized CD8’ peritoneal exudate lymphocytes (PEL-CTLs), express only background levels of perforin/granzymes (Fig. 1 b) and lyse target cells without inflicting the structural membrane lesions (lo-20 nm pores) that are proposed for perforin-mediated lysis. However, expression of even a trace amount of perforin by CTLs may be required (and be sufficient) to mediate lymphocytotoxicity; thus, even as late as 1992, most immunologists (relying on the logic of Ockham’s razor, that the simplest explanation is usually correct) continued to maintain that all lysis mediated by CTL/NK cells requires perforin, granzyme secretion and pore formation.

Tumor Immunity in Perforin-Deficient Mice: A Role for CD95 (Fas/APO-1)

CTL and NK cells use two distinct cytocidal pathways: 1) perforin and granzyme based and 2) CD95L/CD95 mediated. The former requires perforin expression by the effectors (CTL or NK), whereas the latter requires CD95 (Fas/APO-1) expression by the target. We have investigated how these two factors contribute to tumor immune surveillance by studying the immunity of perforin-deficient mice against the progressor C57BL/6 Lewis lung carcinoma 3LL, which expresses no CD95 when cultured in vitro. Unexpectedly, the results indicated that the perforin-independent CD95L/CD95 pathway of CTL/NK plays a role in acting against D122 and Kb39.5 (39.5) high and low metastatic sublines, respectively, derived from the 3LL tumor. Although no membrane-bound CD95 was detected on cultured D122 and 39.5 cells, surface CD95 expression on both D122 and 39.5 was considerably up-regulated when the tumors were grown in vivo. A similarly enhanced expression of CD95 was observed with three additional tumors; LF−, BW, and P815, injected into syngeneic and allogeneic mice. The finding of up-regulated CD95 expression on tumor cells placed in vivo suggests that a CD95-based mechanism plays a role in tumor immunity at early stages of tumor growth. Consequently, the progressive down-regulation of CD95 expression during tumor progression may indeed be an escape mechanism as previously reported. Together, these results suggest a role for CD95-dependent, perforin-independent immunity against certain tumors.

Hypoxia predisposes neonatal rat ventricular myocytes to apoptosis induced by activation of the Fas (CD95/Apo-1) receptor: Fas activation and apoptosis in hypoxic myocytes

OBJECTIVE Since apoptosis is an important contributor to heart diseases in which ischemia and hypoxia are key elements, we tested the hypothesis that hypoxia predisposes neonatal rat ventricular myocytes (NRVM) to Fas-mediated apoptosis, by shifting the balance between antiapoptotic and proapoptotic proteins towards the latter. METHODS Normoxic or hypoxic (22 h, 1% O(2)) cultured NRVM were exposed to recombinant Fas L (rFasL) for 7 h, and apoptosis measured thereafter. RESULTS Whereas in normoxic NRVM, rFasL did not cause apoptosis measured by the TUNEL assay (4.8+/-0.5% in control versus 4.5+/-0.9% in rFasL), in hypoxic cultures rFasL increased the background apoptosis level by 100%. That Fas was functional in normoxic NRVM, despite its inability to mediate apoptosis, was evidenced by the finding that Fas activation increased the diastolic [Ca(2+)](i) levels measured by Fura 2 fluorescence, and caused arrhythmias. In support of our working hypothesis, hypoxia increased Fas expression by 200% (measured by quantitative Western blot), and the expression of the proapoptotic proteins ARTS and FADD by 323 and 250%, respectively, and decreased the expression of the antiapoptotic proteins ARC and FLIP by 90 and 60%, respectively. CONCLUSION By upregulating Fas expression and key proapoptotic proteins, and by downregulating antiapoptotic proteins, hypoxia predisposes ventricular myocytes to Fas-induced apoptosis.