doi: 10.1161/CIRCRESAHA.107.158238
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© 2007 American Heart Association, Inc.
Editorials |
Crosstalk Between Cytotoxic T-Lymphocyte–Associated Antigen-4 and Interleukin-12 in Cytotoxic T-Lymphocyte–Mediated Myocarditis
Adding Another Link to the Chain
From the Nephrology (S.S.A., C.A.E.) and Cardiology (M.L.L.) Divisions, Department of Medicine, University of Texas Health Science Center, San Antonio.
Correspondence to Merry L. Lindsey, PhD, Assistant Professor, Medicine/Cardiology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900. E-mail lindseym{at}uthscsa.edu
See related article, pages 248–257
Key Words: myocarditis • lymphocyte • CTLA-4 • mice
Myocarditis, or inflammation of the myocardium, may arise from immune-mediated destruction of myocardium harboring foreign viral or microbial antigens or from a dysregulated immune system that is unable to distinguish self myocardial antigens from foreign antigens. Of these causes, immune-mediated myocarditis is a primary etiology. Although myocarditis remains a relatively rare event, prognosis following diagnosis remains poor, and 6 year mortality rates approaching 30% have been reported.1 T lymphocytes mediate acquired immunity against a foreign antigen via humoral or cell-mediated mechanisms and hence may affect the outcome of myocarditis. T-lymphocyte involvement was first shown in the 1970s, when T-lymphocyte depletion decreased the inflammatory response and increased survival in a murine model of Coxsackie virus B3–induced myocarditis.2 Since then, research has focused on delineating the roles of individual lymphocyte subtypes. In this issue, Love et al examine the role of cytotoxic T-lymphocyte–associated antigen (CTLA)-4 in CD8+ T-cell regulation by using a transgenic model of CD8+ T-lymphocyte–mediated myocarditis.3
One subset of T lymphocytes is the CD8+ T lymphocytes, which are antigen-specific cytotoxic cells that normally protect against infection by killing infected cells via effector molecules such as perforins and granzymes. In the myocardium, CD8+ lymphocytes regulate both myocarditis and allograft rejection by differentiating into cytotoxic T lymphocytes (CTLs).4,5 Differentiation involves several effector functions, including increased proliferation and cytokine production.6
The CTL response to an infection can be divided into 4 phases. The first is the activation phase, in which naive CTL precursors are primed, undergo cell expansion, acquire effector function, travel to sites of infection, and mediate pathogen clearance. During the second phase, 90% to 95% of effector CTLs die; during the third phase, few memory CTLs are maintained; and during the fourth phase, if antigen reexposure exists, a rapid recall response of memory CTLs occurs.6
CD8+ T lymphocyte activation involves 3 stages, termed signals 1 through 3 (Figure). During signal 1, the T-cell receptor on the lymphocyte binds to the major histocompatibility complex I on the antigen-presenting cell (APC); during signal 2, CD28 on the lymphocyte binds to B7 (CD80/86) on the APC, a process that induces proliferation signals; and during signal 3, interleukin (IL)-12 is released from APC and binds to IL-12 receptors on the T lymphocyte, promoting CTL differentiation.7 CTLA-4 is a factor that has been shown to negatively regulate CD4+ T lymphocytes by competitively binding to B7 and preventing signal 2.
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Testing the hypothesis that the removal of CTLA-4 would enhance myocarditis and overcome the necessity for IL-12 for full CTL differentiation, Love et al3 compared the characteristics of wild-type and CTLA-4–null CD8+ ova-specific (OT-1) lymphocytes generated in the presence or absence of IL-12. They demonstrated in vitro that T cells differentiated with IL-12 killed more SIINFEKL-pulsed target cells, regardless of whether CTLA-4 was present (Table). CTLA-4–deficient CTLs were more efficient at inducing lethality, compared with control wild-type CTLs.
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For in vivo correlations, the mouse model used in this study was the cMy-mOva mouse, which expresses ovalbumin in only cardiac myocytes.3 In this model, when ova peptide–specific CD8+ T lymphocytes are adoptively transferred into the cMy-mOva mice, effector T cells home to the myocardium and induce a robust inflammatory response. Using this model, the authors show that transplant of IL-12 prestimulated CTLA-4–/– OT-1 cells (CTLA-4–/– Tc12) induces a severe myocarditis characterized by cell infiltration, high serum troponin I levels, and very low survival when compared with IL-12–prestimulated CTLA-4+/+ OT-1 cells (CTLA-4+/+ Tc12). Importantly, Love et al also show that nonstimulated CTLA-4–/– OT-1–transplanted cells (CTLA-4–/–Tc0) can induced a mild disease characterized by less cell infiltration and less granzyme production (Table).3
Based on these results, the authors conclude that whereas CTLA-4 limits CTL proliferation and pathogenicity, IL-12 promotes and is necessary for full CTL differentiation in this model. These results suggest that inhibiting CTLA-4 in combination with IL-12 may actually enhance CTL effector function and worsen outcomes. Although these results are applicable for CTL responses to tissue antigens, whether the CTL response to viral or other microbial antigens is applicable remains to be tested.
Although this study adds important insights to the current knowledge with regard to the role of cytotoxic T lymphocytes in myocarditis, several questions are also raised by these results. For example, because myocarditis can occur through other noncytotoxic immune pathways, the role of other cell types needs to be more clearly established. In this case, the natural killer cells might be potentially involved; ie, in the context of the findings of Love et al, because dual labeling was not performed, it is possible that not all 5-bromodeoxyuridine–positive cells were CD8+ lymphocytes. Exactly which cells are proliferating remains to be determined.
Additionally, it is not entirely clear why CTLA-4–/– CTLs are more pathogenic than CTLA-4+/+ cells. More details about why there was increased lethality after adoptive transfer of IL-12–stimulated CTLA-4–/– CTLs requires investigation. Changes in death rates that are attributable to increases in the incidences of heart failure versus arrhythmias versus systemic inflammation would provide further mechanistic clues for clarifying the pathogenic role of CTLs in this disease.
Curtsinger et al examined the requirement for signal 3 in terms of proliferation, effector function, and establishment of a responsive memory population.8 They found that CTL proliferation is proportional to levels of antigen exposure; however, in the absence of IL-12, CD8+ lymphocytes are unable to develop lytic effector functions, even after secondary challenges.8 The report by Love et al further confirms the important role of IL-12 in CTL antigen-specific differentiation.3
An alternative explanation for the phenotype observed by Love et al would be the absence of regulatory T cells (Tregs), which express CD4+CD25+ and function to suppress immunity to infections, tumors, and autoimmune response. Tregs constitutively express CTLA-4, and their interactions with B7–1/2 molecules on activated effector CD8+ cells would be required for immune suppression. The addition of CTLA-expressing Tregs may have potentially rescued the severe myocarditis observed in cMy-mOva mice that received CTLA-4–/–Tc12 OT-1 cells.
On the other hand, CTLA-4 is an inhibitory receptor on T cells, and deficiency in mice is associated with alteration in immune system, with loss of tolerance, expansion of T helper cells, lethal lymphoproliferation, and susceptibility to autoimmune disease.9–11 Indeed, several recent human clinical trials using CTLA-4 blockade to enhance antitumor immunity have been associated with autoimmunity.12,13 Clearly, several avenues of research require further definition before we have a better understanding of the processes involved. The current study adds another dimension to what is known by elucidating an interaction between CTLA-4 and IL-12 in regulating CTL function during myocarditis.
In summary, Love et al have very elegantly demonstrated that CTLA-4 negatively regulates antigen-specific cytotoxic T-lymphocyte proliferation in the context of myocarditis and have added a new step in the complexity of lymphocyte involvement in myocarditis. These results stimulate new areas of research and also provide insight necessary for the effective design of a novel roadmap using immunotherapy approaches.
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Acknowledgments |
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Sources of Funding
We acknowledge support from NIH grants AR052755 (to S.S.A.) and HL75360 (to M.L.L.).
Disclosures
None.
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References |
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Related Article:
Circ. Res. 2007 101: 248-257.
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