Perforin-independent rejection of transplanted human stem cells

The NOD/SCID mouse model is one of the most established model systems for the analysis of human stem cells in vivo. The lack of mature B and T cells renders NOD/SCID mice susceptible to transplantable human stem and progenitor cells. One remaining functional component of the immune system in NOD/SCID mice is natural killer (NK) cells. We rationalized that by eliminating NK cell-mediated cytotoxicity in this model system engraftment of human haematopoietic stem cells could be improved. Thus perforin-deficient NOD/SCID mice (PNOD/SCID) were generated, which display a complete lack of NK cell-mediated cytotoxicity. To test the engraftment potential of human stem cells in PNOD/SCID mice, we compared the repopulating potential of human haematopoietic stem cells in these mice with the repopulating potential in NOD/SCID mice. Upon injection with varying numbers of mononuclear cells from human cord blood, the number of engrafted PNOD/SCID mice was lower (34.8%) than the number of engrafted NOD/SCID mice (64.7%). Similarly, injection of purified CD34(+) human cord blood cells led to engraftment in 32.3% PNOD/SCID versus 60% in NOD/SCID mice. Surprisingly, these results show that the inactivation of cytotoxic activity of NK cells in PNOD/SCID mice did not result in better engraftment with human haematopoietic stem cells. A potential reason for this observation could be that compensatory activation of NK cells in PNOD/SCID mice induces high levels of soluble factors resulting in an environment unfavourable for human stem cell engraftment.

Differential contribution of Fas- and perforin-mediated mechanisms to the cell-mediated cytotoxic activity of naive and in vivo-primed intestinal intraepithelial lymphocytes

Intestinal intraepithelial lymphocytes (IELs) are known to exert strong constitutive cytotoxic activity. In the present study we compared the Ag-specific cytotoxic activity and the effector mechanisms involved in non-Ag-primed, naive and in in vivo-primed IELs and splenic CD8 T cells. Ex vivo isolated naive CD8alphaalpha TCRalphabeta IELs, CD8alphabeta IELs, and splenocytes from lymphocytic choriomeningitis virus (LCMV)-specific TCR transgenic mice exert Ag-specific cytotoxic activity in a long-term, but not in a short-term, cytotoxicity assay. This cytotoxic activity is mainly Fas-Fas ligand mediated and is significantly reduced in the presence of 20 microg/ml Fas-Fcgamma1 fusion protein. Both CD8alphabeta IELs and CD8alphabeta splenocytes isolated from LCMV-infected C57BL/6 mice exert potent perforin-dependent cell-mediated cytotoxicity. CD8alphaalpha TCRalphabeta IELs from LCMV-infected animals, however, show only minimal Ag-specific cytotoxicity. The potent cytotoxic activity of in vivo activated CD8alphabeta IELs is not affected by the addition of Fas-Fcgamma1. Nevertheless CD8alphabeta IELs from LCMV-infected perforin-deficient mice exert Ag-specific cytotoxicity in a short-term cytotoxicity assay, and this cytotoxicity is almost completely blocked by the addition of Fas-Fcgamma1. These results demonstrate that naive CD8alphabeta IELs exert Ag-specific, Fas-Fas ligand-mediated, constitutive cytotoxic activity in a long-term cytotoxicity assay, whereas primed CD8alphabeta IELs primarily use the perforin-dependent exocytosis pathway to exert their potent cytotoxic activity. Furthermore, these results clearly illustrate the requirement for Ag-specific determination of IEL-mediated cytotoxicity, because the elevated, but variable, frequencies of memory-type T cells in this compartment may lead to ambiguous results when polyclonal activation or redirected assays are used.

Homeostatic regulation of CD8+ T cells by perforin

To prevent uncontrolled expansion, the massive proliferation of T cells during an acute immune response has to be followed by controlled deletion. Here we show that similar to Fas, perforin is not only an important effector molecule of cytotoxic T lymphocytes (CTL) but also involved in down-regulating peripheral T cells. Mice deficient for both the CTL effector molecule perforin and the apoptosis-inducing Fas ligand spontaneously develop infiltration of highly activated CD8(+) T cells in kidney and liver and die between 5 and 12 weeks of age. Injection of staphylococcal enterotoxin B (SEB) into perforin-deficient mice results in dramatically increased selective expansion and prolonged persistence of CD8(+), but not CD4(+), SEB-reactive T cells. Also, secondary immunization of TCR transgenic perforin-deficient mice with the lymphocytic choriomeningitis virus glycoprotein-derived epitope peptide leads to an increased proliferation of transgenic CD8(+) T cells, that is not explained by failure to deplete professional antigen-presenting cells. These results point to a novel mechanism of T cell homeostasis in which the acquisition of perforin-dependent cytotoxic activity regulates the expansion and persistence of CD8(+) effector T cells in vivo.

Interleukin 13 is secreted by and stimulates the growth of Hodgkin and Reed-Sternberg cells

Gene expression patterns can provide vital clues to the pathogenesis of neoplastic diseases. We investigated the expression of 950 genes in Hodgkin's disease (HD) by analyzing differential mRNA expression using microarrays. In two independent microarray experiments, the HD-derived cell lines L428 and KMH2 were compared with an Epstein-Barr virus (EBV)-immortalized lymphoblastoid B cell line, LCL-GK. Interleukin (IL)-13 and IL-5 were found to be highly expressed in the HD-derived cell lines. Examination of IL-13 and IL-5 expression by Northern blot analysis and enzyme-linked immunosorbent assay confirmed these results and revealed the expression of IL-13 in a third HD-derived cell line, HDLM2. Control LCL and EBV-negative non-Hodgkin lymphoma-derived cell lines did not express IL-13. In situ hybridization of lymph node tissue from HD patients showed that elevated levels of IL-13 were specifically expressed by Hodgkin/Reed-Sternberg (H/RS) tumor cells. Treatment of a HD-derived cell line with a neutralizing antibody to IL-13 resulted in a dose-dependent inhibition of H/RS cell proliferation. These data suggest that H/RS cells produce IL-13 and that IL-13 plays an important role in the stimulation of H/RS cell growth, possibly by an autocrine mechanism. Modulation of the IL-13 signaling pathway may be a logical objective for future therapeutic strategies.

TNF receptor 1-dependent beta cell toxicity as an effector pathway in autoimmune diabetes

Autoimmune diabetes is characterized by a chronic progressive inflammatory autoimmune reaction that ultimately causes the selective elimination of pancreatic beta cells. To address the question of whether the cell death-inducing cytokines TNF and lymphotoxin alpha are involved in this process, we generated nonobese diabetic (NOD) mice that are deficient for TNF receptor 1 (TNFR1 or TNFRp55). Insulitis developed in these mice similarly to that in normal control NOD mice, but progression to diabetes was completely abrogated. Since this was probably due to the complex immunomodulatory effects of TNF and lymphotoxin alpha signaled via TNFR1 on lymphohemopoietic cells, adoptive transfer experiments with spleen cells from diabetic NOD mice were conducted. It was found that the absence of TNFR1 in recipients delayed diabetes induced by normal control and precluded diabetes induced by perforin-deficient spleen cells. In a CD8+ T cell-mediated model of diabetes, however, diabetes induced by adoptive transfer of TCR transgenic lymphocytic choriomeningitis virus glycoprotein-specific CD8+ T cells was not delayed by the absence of TNFR1 in recipient mice. Together with the described expression patterns of perforin and TNF in the mononuclear islet infiltrates of NOD mice, these results indicate that two diabetogenic effector mechanisms are delivered by distinct cell populations: CD8+ T cells lyse beta cells via perforin-dependent cytotoxicity, whereas CD4+ T cells, macrophages, and dendritic cells contribute to diabetes development via TNFR1-dependent beta cell toxicity.

Cytotoxicity is mandatory for CD8(+) T cell-mediated contact hypersensitivity

Contact hypersensitivity (CHS) is a T cell-mediated skin inflammation induced by epicutaneous exposure to haptens in sensitized individuals. We have previously reported that CHS to dinitrofluorobenzene in mice is mediated by major histocompatibility complex (MHC) class I-restricted CD8(+) T cells. In this study, we show that CD8(+) T cells mediate the skin inflammation through their cytotoxic activity. The contribution of specific cytotoxic T lymphocytes (CTLs) to the CHS reaction was examined both in vivo and in vitro, using mice deficient in perforin and/or Fas/Fas ligand (FasL) pathways involved in cytotoxicity. Mice double deficient in perforin and FasL were able to develop hapten-specific CD8(+) T cells in the lymphoid organs but did not show CHS reaction. However, they did not generate hapten-specific CTLs, demonstrating that the CHS reaction is dependent on cytotoxic activity. In contrast, Fas-deficient lpr mice, FasL-deficient gld mice, and perforin-deficient mice developed a normal CHS reaction and were able to generate hapten-specific CTLs, suggesting that CHS requires either the Fas/FasL or the perforin pathway. This was confirmed by in vitro studies showing that the hapten-specific CTL activity was exclusively mediated by MHC class I-restricted CD8(+) T cells which could use either the perforin or the Fas/FasL pathway for their lytic activity. Thus, cytotoxic CD8(+) T cells, commonly implicated in the host defence against tumors and viral infections, could also mediate harmful delayed-type hypersensitivity reactions.

Aplastic anemia rescued by exhaustion of cytokine-secreting CD8+ T cells in persistent infection with lymphocytic choriomeningitis virus

Aplastic anemia may be associated with persistent viral infections that result from failure of the immune system to control virus. To evaluate the effects on hematopoiesis exerted by sustained viral replication in the presence of activated T cells, blood values and bone marrow (BM) function were analyzed in chronic infection with lymphocytic choriomeningitis virus (LCMV) in perforin-deficient (P0/0) mice. These mice exhibit a vigorous T cell response, but are unable to eliminate the virus. Within 14 d after infection, a progressive pancytopenia developed that eventually was lethal due to agranulocytosis and thrombocytopenia correlating with an increasing loss of morphologically differentiated, pluripotent, and committed progenitors in the BM. This hematopoietic disease caused by a noncytopathic chronic virus infection was prevented by depletion of CD8+, but not of CD4+, T cells and accelerated by increasing the frequency of LCMV-specific CD8+ T cells in T cell receptor (TCR) transgenic (tg) mice. LCMV and CD8+ T cells were found only transiently in the BM of infected wild-type mice. In contrast, increased numbers of CD8+ T cells and LCMV persisted at high levels in antigen-presenting cells of infected P0/0 and P0/0 x TCR tg mice. No cognate interaction between the TCR and hematopoietic progenitors presenting either LCMV-derived or self-antigens on the major histocompatibility complex was found, but damage to hematopoiesis was due to excessive secretion and action of tumor necrosis factor (TNF)/lymphotoxin (LT)-alpha and interferon (IFN)-gamma produced by CD8+ T cells. This was studied in double-knockout mice that were genetically deficient in perforin and TNF receptor type 1. Compared with P0/0 mice, these mice had identical T cell compartments and T cell responses to LCMV, yet they survived LCMV infection and became life-long virus carriers. The numbers of hematopoietic precursors in the BM were increased compared with P0/0 mice after LCMV infection, although transient blood disease was still noticed. This residual disease activity was found to depend on IFN-gamma-producing LCMV-specific T cells and the time point of hematopoietic recovery paralleled disappearance of these virus-specific, IFN-gamma-producing CD8+ T cells. Thus, in the absence of IFN-gamma and/or TNF/LT-alpha, exhaustion of virus-specific T cells was not hampered.

Theiler's virus infection of perforin-deficient mice

Theiler's virus, a murine picornavirus, infects the central nervous systems of C57BL/6 mice and is cleared after approximately 10 days by a process which requires CD8+ cytotoxic T cells. We used perforin-deficient C57BL/6 mice to test the role of this protein in viral clearance. Perforin-deficient mice died from viral encephalomyelitis between days 12 and 18 postinoculation. They had high levels of viral RNA in their central nervous systems until the time of death. In contrast, viral RNA had disappeared by day 11 postinoculation in wild-type C57BL/6 mice. Cytotoxic T cells can kill infected cells by two main mechanisms: the secretion of the pore-forming protein perforin or the interaction of the Fas ligand with the apoptosis-inducing Fas molecule on the target cell. Our results demonstrate that clearance of Theiler's virus from the central nervous system in C57BL/6 mice is perforin dependent.

Essential contribution of caspase 3/CPP32 to apoptosis and its associated nuclear changes

Caspases are fundamental components of the mammalian apoptotic machinery, but the precise contribution of individual caspases is controversial. CPP32 (caspase 3) is a prototypical caspase that becomes activated during apoptosis. In this study, we took a comprehensive approach to examining the role of CPP32 in apoptosis using mice, embryonic stem (ES) cells, and mouse embryonic fibroblasts (MEFs) deficient for CPP32. CPP32(ex3-/-) mice have reduced viability and, consistent with an earlier report, display defective neuronal apoptosis and neurological defects. Inactivation of CPP32 dramatically reduces apoptosis in diverse settings, including activation-induced cell death (AICD) of peripheral T cells, as well as chemotherapy-induced apoptosis of oncogenically transformed CPP32(-/-) MEFs. As well, the requirement for CPP32 can be remarkably stimulus-dependent: In ES cells, CPP32 is necessary for efficient apoptosis following UV- but not gamma-irradiation. Conversely, the same stimulus can show a tissue-specific dependence on CPP32: Hence, TNFalpha treatment induces normal levels of apoptosis in CPP32 deficient thymocytes, but defective apoptosis in oncogenically transformed MEFs. Finally, in some settings, CPP32 is required for certain apoptotic events but not others: Select CPP32(ex3-/-) cell types undergoing cell death are incapable of chromatin condensation and DNA degradation, but display other hallmarks of apoptosis. Together, these results indicate that CPP32 is an essential component in apoptotic events that is remarkably system- and stimulus-dependent. Consequently, drugs that inhibit CPP32 may preferentially disrupt specific forms of cell death.

Altered peptide ligands trigger perforin- rather than Fas-dependent cell lysis

CTLs lyse Fas-expressing target cells by the concomitant action of a perforin- and a Fas-dependent mechanism. This study analyzed whether target cells pulsed with T cell antagonists and other altered peptide ligands (APLs) were susceptible selectively to only one of these two mechanisms. In vivo and in vitro activated T cells from transgenic mice expressing a TCR specific for lymphocytic choriomeningitis virus were used as effector cells. To distinguish between perforin- and Fas-dependent cytotoxicity, T cells from normal or perforin-deficient mice were used to lyse peptide-pulsed Fas-positive or Fas-negative target cells. In contrast to previous reports that have shown that APLs selectively induce the Fas-dependent pathway of cytotoxicity, our results demonstrate that target cells pulsed with T cell antagonists and other APLs are lysed predominantly by the perforin-dependent pathway. The contribution of Fas-mediated cytotoxicity was similar for the full agonist and the APLs. Thus, full agonists, partial agonists, and antagonists trigger similar and not distinct pathways of cytotoxicity.

Altered peptide ligands trigger perforin- rather than Fas-dependent cell lysis

CTLs lyse Fas-expressing target cells by the concomitant action of a perforin- and a Fas-dependent mechanism. This study analyzed whether target cells pulsed with T cell antagonists and other altered peptide ligands (APLs) were susceptible selectively to only one of these two mechanisms. In vivo and in vitro activated T cells from transgenic mice expressing a TCR specific for lymphocytic choriomeningitis virus were used as effector cells. To distinguish between perforin- and Fas-dependent cytotoxicity, T cells from normal or perforin-deficient mice were used to lyse peptide-pulsed Fas-positive or Fas-negative target cells. In contrast to previous reports that have shown that APLs selectively induce the Fas-dependent pathway of cytotoxicity, our results demonstrate that target cells pulsed with T cell antagonists and other APLs are lysed predominantly by the perforin-dependent pathway. The contribution of Fas-mediated cytotoxicity was similar for the full agonist and the APLs. Thus, full agonists, partial agonists, and antagonists trigger similar and not distinct pathways of cytotoxicity.

Reduced incidence and delayed onset of diabetes in perforin-deficient nonobese diabetic mice

To investigate the role of T cell-mediated, perforin-dependent cytotoxicity in autoimmune diabetes, perforin-deficient mice were backcrossed with the nonobese diabetes mouse strain. It was found that the incidence of spontaneous diabetes over a 1 yr period was reduced from 77% in perforin +/+ control to 16% in perforin-deficient mice. Also, the disease onset was markedly delayed (median onset of 39.5 versus 19 wk) in the latter. Insulitis with infiltration of CD4(+) and CD8(+) T cells occurred similarly in both groups of animals. Lower incidence and delayed disease onset were also evident in perforin-deficient mice when diabetes was induced by cyclophosphamide injection. Thus, perforin-dependent cytotoxicity is a crucial effector mechanism for beta cell elimination by cytotoxic T cells in autoimmune diabetes. However, in the absence of perforin chronic inflammation of the islets can lead to diabetogenic beta cell loss by less efficient secondary effector mechanisms.

CD8Tc1 and Tc2 cells secrete distinct cytokine patterns in vitro and in vivo but induce similar inflammatory reactions

Naive CD8 T cells, similar to CD4 T cells, can differentiate into at least two subsets of cytolytic effector cells with distinct cytokine patterns: T cytotoxic-1 (Tc1) cells secrete a Th1-like cytokine pattern, including IL-2 and IFN-gamma; and Tc2 cells produce Th2 cytokines, including IL-4, IL-5, and IL-10. As CD4 Th1 cells induce delayed-type hypersensitivity (DTH) more effectively than Th2 cells, we tested the potential ability of Tc1 and Tc2 cells to induce DTH. Allospecific Tc1 or Tc2 cells were injected into the footpads of naive mice expressing the target Ag. Tc1 and Tc2 cells induced comparable levels of Ag-specific footpad swelling with similar kinetics. They also induced similar levels of footpad edema and similar infiltration of macrophages and neutrophils. However, Tc2 cells induced slightly more eosinophil infiltration. Analysis of footpad extracts showed that Tc1 and Tc2 cells retained their distinct in vitro cytokine profiles in the injected footpads. These results suggest that both Tc1 and Tc2 cytokines can be associated with the DTH reaction induced by CD8 T cells. Perforin-deficient Tc1 or Tc2 cells also induced DTH, although at lower levels, suggesting that perforin-mediated cytotoxicity of CD8 T cells is not essential for CD8-induced DTH. Thus, despite their distinct cytokine profiles in vitro and in vivo, Tc1 and Tc2 cells induce similar DTH reactions.

CD8Tc1 and Tc2 cells secrete distinct cytokine patterns in vitro and in vivo but induce similar inflammatory reactions

Naive CD8 T cells, similar to CD4 T cells, can differentiate into at least two subsets of cytolytic effector cells with distinct cytokine patterns: T cytotoxic-1 (Tc1) cells secrete a Th1-like cytokine pattern, including IL-2 and IFN-gamma; and Tc2 cells produce Th2 cytokines, including IL-4, IL-5, and IL-10. As CD4 Th1 cells induce delayed-type hypersensitivity (DTH) more effectively than Th2 cells, we tested the potential ability of Tc1 and Tc2 cells to induce DTH. Allospecific Tc1 or Tc2 cells were injected into the footpads of naive mice expressing the target Ag. Tc1 and Tc2 cells induced comparable levels of Ag-specific footpad swelling with similar kinetics. They also induced similar levels of footpad edema and similar infiltration of macrophages and neutrophils. However, Tc2 cells induced slightly more eosinophil infiltration. Analysis of footpad extracts showed that Tc1 and Tc2 cells retained their distinct in vitro cytokine profiles in the injected footpads. These results suggest that both Tc1 and Tc2 cytokines can be associated with the DTH reaction induced by CD8 T cells. Perforin-deficient Tc1 or Tc2 cells also induced DTH, although at lower levels, suggesting that perforin-mediated cytotoxicity of CD8 T cells is not essential for CD8-induced DTH. Thus, despite their distinct cytokine profiles in vitro and in vivo, Tc1 and Tc2 cells induce similar DTH reactions.

Cytotoxicity and weak CD40 ligand expression of CD8+ type 2 cytotoxic T cells restricts their potential B cell helper activity

Naive CD8+ T cells differentiate into distinct cytokine-secreting subsets: T helper (Th)1-like cytotoxic T cells (Tc1) and Th2-like Tc2. Although Th2 cells provide strong B cell help, we show that Tc2 cells secreting the same cytokines provide only modest B cell help for IgM production, and only when large numbers of B cells were stimulated with small numbers of Tc2 cells. Lack of effective B cell help by Tc2 cells was attributable partly to their cytotoxicity towards B cells. Both Tc1 and Tc2 cells killed small resting B cells mainly by a perforin-dependent mechanism. In contrast to normal Tc2 cells, perforin-deficient Tc2 cells failed to kill small resting B cells and induced IgM and IgG1 production, although their B cell help was significantly lower than that mediated by Th2 cells. This may be partly attributable to the ability of Tc2 but not Th2 cells to kill activated B cells even in the absence of perforin. Plate-bound anti-CD3 antibodies inhibited Tc2 killing of B cells and induced substantial immunoglobulin production. Additionally, Tc1 and Tc2 cells failed to express CD40 ligand (CD40L), whereas Th1 and Th2 cells expressed high levels of CD40L. Stimulation of Tc1 and Tc2 cells with plate-bound anti-CD3 antibodies for extended periods resulted in low-level expression of CD40L. Proliferation of small resting B cells correlated with immunoglobulin production: proliferation was promoted strongly by Th1 and Th2, weakly by normal Tc1 and Tc2, and moderately by perforin-deficient Tc1 and Tc2 cells. Thus, Tc2 cells may not contribute significantly to cognate B cell help during normal responses.

Control of immunodeficiency and lymphoproliferation in mouse AIDS: studies of mice deficient in CD8+ T cells or perforin

CD8+ T cells were previously shown to be important in preventing lymphoproliferation and immunodeficiency following infection of murine AIDS (MAIDS)-resistant mice with the LP-BM5 mixture of murine leukemia viruses. To further evaluate the mechanisms contributing to MAIDS resistance, we studied mice lacking CD8+ T cells or deficient in perforin due to knockout of the beta2-microglobulin (beta2M) or perforin gene, respectively. In contrast to wild-type, MAIDS-resistant controls, B10.A mice homozygous for the beta2M mutation and B10.D2 mice homozygous for the perforin mutation were diagnosed as having MAIDS by 5 to 8 weeks after infection by the criteria of lymphoproliferation, impaired proliferative responses to mitogens, and changes in cell populations as judged by histopathology and flow cytometry. Unexpectedly, there was no progression of lymphoproliferation through 24 weeks, even though immune functions were severely compromised. Expression of the defective virus responsible for MAIDS was enhanced in spleens of the knockouts in comparison with wild-type mice. These results demonstrate that perforin-dependent functions of CD8+ T cells contribute to MAIDS resistance but that other, non-CD8-dependent mechanisms are of equal or greater importance.

The transcription factor interferon regulatory factor-1 is essential for natural killer cell function in vivo

The activation of natural killer (NK) cells, cytotoxic lymphocytes capable of major histocompatibility complex (MHC)-unrestricted killing and early antiviral defense, is temporally related to the increased interferon (IFN)-alpha/beta production that is seen in the viral infection of mice. Type I IFN (IFN-alpha/beta) are expressed in many cell types early after primary viral infection and have been shown to mediate resistance against a variety of viruses. In this study, the role of the transcriptional activator IFN regulatory factor-1 (IRF-1) in murine NK cell activity was assessed. IRF-1-deficient mice displayed a normal frequency of NK marker-positive cells, but exhibited greatly reduced NK cell-mediated cytotoxicity after both virus infection and stimulation with the IFN inducer polyinosinic:polycytidilic acid in vivo. In vitro, cytolytic activity in IRF-1-deficient NK cells remained defective after stimulation with IFN-beta, IL-2, and IL-12. IRF-1-deficient mice were unable to eliminate syngeneic MHC class I-negative tumor cells in vivo, and had a reduced ability to reject parental semi-allogeneic donor cells from the circulation. Thus, IRF-1 is essential for the induction of NK cell-mediated cytotoxicity and for the in vivo effector functions that are mediated by this activity.

Decreased tumor surveillance in perforin-deficient mice

Immune surveillance against tumors usually depends on T cell recognition of tumor antigens presented by major histocompatibility complex (MHC) molecules, whereas MHC class I- tumors may be controlled by natural killer (NK) cells. Perforin-dependent cytotoxicity is a major effector function of CD8+ MHC class I-restricted T cells and of NK cells. Here, we used perforin-deficient C57BL/6 (PKO) mice to study involvement of perforin and Fas ligand in tumor surveillance in vivo. We induced tumors in PKO and normal C57BL/6 mice by (a) injection of different syngeneic tumor cell lines of different tissue origin in naive and primed mice; (b) administration of the chemical carcinogens methylcholanthrene (MCA) or 12-O-tetradecanoylphorbol-13-acetate (TPA) plus 7,12-dimethylbenzanthracene (DMBA), or (c) by injection of acutely oncogenic Moloney sarcoma virus. The first set of models analyzes the defense against a tumor load given at once, whereas the last two sets give information on immune defense against tumors at the very moment of their generation. Most of the tumor cell lines tested were eliminated 10-100-fold better by C57BL/6 mice in an unprimed situation; after priming, the differences were more pronounced. Lymphoma cells transfected with Fas were controlled 10-fold better by PKO and C57BL/6 mice when compared to untransfected control cells, indicating some role for FasL in tumor control. MCA-induced tumors arose more rapidly and with a higher incidence in PKO mice compared to C57BL/6 or CD8-deficient mice. DMBA+TPA-induced skin papillomas arose with similar high incidence and comparable kinetics in both mouse strains. C57BL/6 and PKO mice have a similar incidence of Moloney murine sarcoma and leukemia virus-induced sarcomas, but tumors are larger and regression is retarded in PKO mice. Thus, perforin-dependent cytotoxicity is not only a crucial mechanism of both cytotoxic T lymphocyte- and NK-dependent resistance to injected tumor cell lines, but also operates during viral and chemical carcinogenesis in vivo. Experiments addressing the role of Fas-dependent cytotoxicity by studying resistance to tumor cell lines that were stably transfected with Fas neither provided evidence for a major role of Fas nor excluded a minor contribution of Fas in tumor surveillance.

Perforin and Fas killing by CD8+ T cells limits their cytokine synthesis and proliferation

During an immune response, effector CD8+ T cells can kill infected cells by the perforin-dependent pathway. In comparison to CD4+ T cells, which are major sources of cytokines, normal CD8+ T cells produced less interleukin 2 and interferon gamma, and proliferated less vigorously after antigenic stimulation. Killing of target cells was a major cause of these reduced responses, since perforin-deficient CD8+ T cells showed substantially increased cytokine synthesis and proliferation. Cytotoxicity by the alternate Fas pathway also resulted in self-limitation of CD8+ T cell cytokine synthesis. This relationship between cytotoxicity and cytokine synthesis may regulate CD8+ T function in different phases of an immune response.

Different roles for cytotoxic T cells in the control of infections with cytopathic versus noncytopathic viruses

The assessment of the role of T-cell-mediated cytotoxicity in immunity to viral infections has been difficult to address directly and therefore has been controversial. Recent experiments with perforin-deficient mice have shown that cytotoxicity is crucial for the resolution of infection with lymphocytic choriomeningitis virus but not for the resolution of infection with vaccinia, vesicular stomatitis, Semliki Forest or influenza virus. These findings may reflect the general pattern that T-cell-mediated cytotoxicity is crucial only for the resolution of infections with noncytopathic viruses, whereas infections with cytopathic viruses are mainly controlled by soluble mediators such as antibodies and interferons.

Development of insulitis without diabetes in transgenic mice lacking perforin-dependent cytotoxicity

It is widely accepted that T cells play an important role in the destruction of beta cells leading to autoimmune type I diabetes, but the involved effector mechanisms have remained unclear. We addressed this issue by testing the role of perforin-dependent cytotoxicity in a disease model involving transgenic mice expressing glycoprotein of lymphocytic choriomeningitis virus (LCMV-GP) in the beta cells of the endocrine pancreas. In such mice, LCMV infection leads to a potent LCMV-GP-specific T cell response resulting in rapid development of diabetes. We report here that in perforin-deficient LCMV-GP transgenic mice, LCMV infection failed to induce diabetes despite the activation of LCMV-GP-specific T cells. Deletion of nu beta 6+ T cells in Mls-1a perforin-deficient mice and the activation of LCMV-GP-specific T cells in perforin-deficient LCMV-GP transgenic mice, however, indicated that thymic tolerance induction by negative selection was not affected by the disruption of the perforin gene and that there is no fundamental difference between the T cell repertoires of normal control and perforin-deficient mice. In addition, adoptive transfer of LCMV-GP-specific TCR transgenic perforin-deficient T cells activated by LCMV-GP recombinant vaccinia virus led to marked insulitis with infiltration of CD4+ and CD8+ T cells without the development of diabetes. These findings indicate that perforin-dependent cytotoxicity is not required for the initiation of insulitis but is crucial for the destruction of beta cells in the later phase of the disease process. Other mechanisms or soluble factors present in the inflammatory islet infiltrate apparently lack the ability to efficiently induce diabetogenic beta cell damage.

Molecular mechanisms of lymphocyte-mediated cytotoxicity and their role in immunological protection and pathogenesis in vivo

Studies with perforin-deficient mice have demonstrated that two independent mechanisms account for T cell-mediated cytotoxicity: A main pathway is mediated by the secretion of the pore-forming protein perforin by the cytotoxic T cell, whereas an alternative nonsecretory pathway relies on the interaction of the Fas ligand that is upregulated during T cell activation with the apoptosis-inducing Fas molecule on the target cell. NK cells use the former pathway exclusively. The protective role of the perforin-dependent pathway has been shown for infection with the noncytopathic lymphocytic choriomeningitis virus, for infection with Listeria monocytogenes, and for the elimination of tumor cells by T cells and NK cells. In contrast, perforin-dependent cytotoxicity is not involved in protection against the cytopathic vaccinia virus and vesicular stomatitis virus. LCMV-induced immunopathology and autoimmune diabetes have been found to require perforin-expression. A contribution of perforin-dependent cytotoxicity to the rejection of MHC class I-disparate heart grafts has also been observed. Its absence is efficiently compensated in rejection of fully allogeneic organ or skin grafts. So far, evidence for a role of Fas-dependent cytotoxicity as a T cell effector mechanism in vivo is lacking. Current data suggest that the main function of Fas may be in regulation of the immune response and apparently less at the level of an effector mechanism in host defense. Further analysis is necessary, however, to settle this point finally.

The role of perforin-expression by granular metrial gland cells in pregnancy

The pregnant uterus of humans and rodents contains a population of granulated lymphoid cells, which, in the mouse, are called granular metrial gland (GMG) cells and have been described to express high levels of perforin. Since there is evidence for cytolytic activity of these cells and since perforin is a crucial effector molecule for the lytic action of cytotoxic T cells and natural killer cells, we evaluated the function of perforin in the pregnant uterus by using perforin-deficient mice. Perforin-deficient female mice were found to reproduce as efficiently as normal control females when bred either with syngeneic or allogeneic males. However, perforin-deficient mice differed from normal mice in that the frequency of GMG cells was significantly higher within maternal blood spaces and within several compartments of the feto-maternal interface. Proliferating GMG cells, identified by [3H] thymidine incorporation, were observed during more advanced stages of pregnancy when compared to normal controls. In contrast to normal mice, perforin-deficient mice did not display GMG cells attached to degenerating trophoblasts; instead perforin-deficient GMG cells were often observed in association with small maternal lymphocytes. In addition, the lack of transmission of lymphocytic choriomeningitis virus from infected pregnant perforin-deficient mice to the fetuses argued against a role of perforin expression by GMG cells in prevention of virus transmission from the mother to the fetus. Our data indicate that functional perforin is not necessary for successful pregnancies. The morphological changes in the pregnant uterus of perforin-deficient mice might, however, point to a certain, as-yet undefined function of perforin in the uterus of pregnant normal mice, which is functionally compensated in perforin-deficient mice.

The roles of perforin- and Fas-dependent cytotoxicity in protection against cytopathic and noncytopathic viruses

In vitro, T cell-dependent cytotoxicity is mediated by two distinct mechanisms, one being perforin-, the other Fas-dependent. The contribution of both of these mechanisms to clearance of viral infections was investigated in mice for the non-cytopathic lymphocytic choriomeningitis virus (LCMV) and the cytopathic vaccinia, vesicular stomatitis (VSV) and Semliki forest (SFV) viruses. Clearance of an acute LCMV infection was mediated by the perforin-dependent mechanism without measurable involvement of the Fas-dependent pathway. For the resolution of vaccinia virus infection and for resistance against VSV and SFV, however, neither of the two pathways was required. These data suggest that perforin-dependent cytotoxicity mediated by T cells is crucial for protection against non-cytopathic viruses, whereas infections with cytopathic viruses are controlled by nonlytic T cell-dependent soluble mediators such as cytokines (IFN-gamma against vaccinia virus) and neutralizing antibodies (against VSV and SFV).

Perforin dependence of natural killer cell-mediated tumor control in vivo

Adaptive immune surveillance by T cells against infections and tumors depends on the presence of antigenic peptides presented by major histocompatibility complex (MHC) molecules. If antigenic tumor-specific peptides or MHC class I molecules are absent, the adaptive T cell immune response fails. Natural killer (NK) cells seem to complement the specific T cells by recognizing target cells lacking MHC class I (e.g. RMA-S). The role of perforin, which is crucially involved in T cell and NK cell-mediated target cell lysis, was evaluated in mice lacking perforin with respect to their capacity to eliminate a syngeneic lymphoid tumor. Here, we show that growth of MHC class I RMA-S tumor cells in unprimed mice was controlled by NK cells through perforin-dependent cytotoxicity.

Pathways of tumour cell killing by activated macrophages: both tumour cell membrane and nucleus can be the primary target

Plasma membrane and nucleus can be primary targets of tumour cell killing by activated macrophages (AMø). Necrotic-type cytotoxicity with loss of membrane integrity and cytoplasmic swelling was expressed by AMø from normal and from perforin-deficient mice, indicating that perforin was not involved. Incubation with AMø consistently triggered the release of thymidine from prelabelled targets, whereas chromatin condensation and small DNA fragments were only occasionally detected. It is shown by means of Pulsed-Field Gel Electrophoresis that DNA degradation in target cells is a slowly progressing process that may stop at any time, indicating that nuclear-type killing doesnot necessarily lead to the formation of low molecular weight fragments. Neither Fas nor the p55 tumour necrosis factor receptor appear to be involved in signalling nuclear-type killing. Accordingly, AMø do mediate membrane- and nuclear-type killing but the mechanisms differ from those identified in T cell cytotoxicity.

Acute rejection of vascular heart allografts by perforin-deficient mice

To study the role of perforin in cell-mediated graft rejection, vascularized hearts were grafted to perforin-deficient C57BL/6 and control C57BL/6 recipient mice. Fully allogeneic heart grafts (BALB/c) were acutely rejected by both recipients within 6 days. Peritoneal exudate lymphocytes from control mice but not from perforin-deficient mice exhibit a strong alloreactive cytotoxic activity in vitro. Histological analysis of the rejected tissues demonstrated extensive mononuclear cell infiltrates in both recipients. Flow cytometry analysis and immunohistology of graft-infiltrating cells showed similar proportions of lymphocyte subsets (CD8 >> CD4). Collectively, these data indicate that perforin is not essential in the cell-mediated acute rejection of a fully mismatched heart allograft. However, perforin-dependent effector mechanisms appeared to be limiting in the T cell-mediated rejection of heart allografts differing only at a single major histocompatibility complex class I antigen (bm1), because these grafts survived longer (mean 87.8 days) in perforin-deficient than in control mice (mean 31.5 days).

CD8+ T cell-mediated protection against an intracellular bacterium by perforin-dependent cytotoxicity

Growth of Listeria monocytogenes is mainly controlled by macrophages, which are activated by specific T cells. A potential role of CD8+ T cells by direct lysis of infected cells was investigated in perforin-deficient mice generated by homologous recombination. The absence of perforin-mediated cytotoxicity resulted in delayed clearance of Listeria from the spleen but not the liver after primary infection, overall susceptibility to Listeria however was not increased. Protection against a secondary infection was drastically impaired in perforin-deficient mice. Adoptive transfer of immune spleen cells to recipients revealed that anti-Listeria protection by CD8+ T cells from perforin-deficient versus normal mice was about 10-fold reduced in livers and about 100-fold reduced in the spleen of recipients. CD4+ T cells from immune control and perforin-deficient mice conferred comparable protection. These results indicate that the protective effect of CD8+ T cells against an intracellular bacterium mainly evident in secondary infection is mediated by a perforin-dependent pathway, presumably cytotoxicity, and less by other direct or indirect effector mechanisms.

Fas and perforin pathways as major mechanisms of T cell-mediated cytotoxicity

Two molecular mechanisms of T cell-mediated cytotoxicity, one perforin-based, the other Fas-based, have been demonstrated. To determine the extent of their contribution to T cell-mediated cytotoxicity, a range of effector cells from normal control or perforin-deficient mice were tested against a panel of target cells with various levels of Fas expression. All cytotoxicity observed was due to either of these mechanisms, and no third mechanism was detected. Thus, the perforin- and Fas-based mechanisms may account for all T cell-mediated cytotoxicity in short-term in vitro assays.

Cytotoxicity mediated by T cells and natural killer cells is greatly impaired in perforin-deficient mice

Perforin-deficient mice have been generated by homologous recombination to determine whether the effects of CD8+ cytolytic T cells and natural killer cells are mediated by pore formation involving perforin. These mice are viable and fertile and have normal numbers of CD8+ T cells and natural killer cells which do not lyse virus-infected or allogeneic fibroblasts or natural killer target cells in vitro. The mice fail to clear lymphocytic choriomeningitis virus and they eliminate fibrosarcoma tumour cells with reduced efficiency. Perforin is therefore a key effector molecule for T-cell- and natural killer-cell-mediated cytolysis.

Morphologic analysis of human lymphokine-activated killer (LAK) cells

Lymphokine-activated killer (LAK) cells obtained from normal donors at various days of in vitro cultivation have been studied by several methods including scanning (SEM) and transmission (TEM) electron microscopy, immuno-electron microscopy, in situ hybridization and flow cytometric DNA measurements. In addition, the cytotoxic activity of LAK cells against several tumor cells was examined by 51Cr-release assay and by SEM and TEM. The LAK cells displayed a uniform ultrastructural appearance concerning surface structure and morphology of organelles. They contained typical lysosomal granules which by immuno-electron microscopy showed a specific localization of perforin I (PI). The presence of PI and granzymeA mRNA in the cytoplasm was confirmed by in situ hybridization using specific antisense probes. Frequency and increased of specific mRNA-containing cells was similar for both genes. Single LAK cells were further characterized by peculiar nuclear inclusion bodies (IB) which were presumably formed by trapped profiles of endoplasmic reticulum. Flow cytometric analysis revealed normal DNA content of LAK cells even after prolonged cultivation indicating that the IB were not associated with aneuploidy of the effector cells. The LAK cells were highly effective in lysing K562 and DAUDI cells as shown by 51Cr-release assay. They caused characteristic morphologic alterations of target cells similar to those found in cytotoxic T-lymphocyte (CTL) and NK-cell-mediated cytolysis. SEM and TEM studies on specimens prepared by routine procedures or by cryopreparation showed that the tumor cell membrane was the initial target for the LAK cell attack whereas other cell compartments were damaged only in advanced stages of cytolysis. Summarizing our study demonstrates that LAK cells have a characteristic ultrastructure which in some aspects differs from that of CTL and NK cells, and that LAK cells appear to destroy tumor cells by mechanisms similar to those of other cytotoxic effector cells.

Detection of perforin and granzyme A mRNA in infiltrating cells during infection of mice with lymphocytic choriomeningitis virus

The analysis of gene expression in cytotoxic T cells by in situ hybridization of serial liver and brain sections from mice infected with lymphocytic choriomeningitis virus (LCMV) and immunostaining with T cell marker- and virus-specific antibodies revealed a close histological association of infiltrating lymphocytes expressing the perforin and granzyme A genes with virally infected cells. Maximal frequency of perforin and granzyme A mRNA-containing cells on liver sections preceded by about 2 days maximal LCMV-specific cytotoxicity of the lymphoid liver infiltrating cells. These results are most consistent with an involvement of perforin and granzyme A in cell-mediated cytotoxicity in vivo.