Ngozi Monu

Signaling defects in anti-tumor T cells

Summary: The immune response to cancer has been long recognized, including both innate and adaptive responses, showing that the immune system can recognize protein products of genetic and epigenetic changes in transformed cells. The accumulation of antigen‐specific T cells within the tumor, the draining lymph node, and the circulation, either in newly diagnosed patients or resultant from experimental immunotherapy, proves that tumors produce antigens and that priming occurs. Unfortunately, just as obviously, tumors grow, implying that anti‐tumor immune responses are either not sufficiently vigorous to eliminate the cancer or that anti‐tumor immunity is suppressed. Both possibilities are supported by current data. In experimental animal models of cancer and also in patients, systemic immunity is usually not dramatically suppressed, because tumor‐bearing animals and patients develop T‐cell‐dependent immune responses to microbes and to either model antigens or experimental cancer vaccines. However, inhibition of specific anti‐tumor immunity is common, and several possible explanations of tolerance to tumor antigens or tumor‐induced immunesuppression have been proposed. Inhibition of effective anti‐tumor immunity results from the tumor or the host response to tumor growth, inhibiting the activation, differentiation, or function of anti‐tumor immune cells. As a consequence, anti‐tumor T cells cannot respond productively to developmental, targeting, or activation cues. While able to enhance the number and phenotype of anti‐tumor T cells, the modest success of immunotherapy has shown the necessity to attempt to reverse tolerance in anti‐tumor T cells, and the vanguard of experimental therapy now focuses on vaccination in combination with blockade of immunosuppressive mechanisms. This review discusses several potential mechanisms by which anti‐tumor T cells may be inhibited in function.

Defective proximal TCR signaling inhibits CD8+ tumor-infiltrating lymphocyte lytic function.

CD8+ tumor-infiltrating lymphocytes (TIL) are severely deficient in cytolysis, a defect that may permit tumor escape from immune-mediated destruction. Because lytic function is dependent upon TCR signaling, we have tested the hypothesis that primary TIL have defective signaling by analysis of the localization and activation status of TIL proteins important in TCR-mediated signaling. Upon conjugate formation with cognate target cells in vitro, TIL do not recruit granzyme B+ granules, the microtubule-organizing center, F-actin, Wiskott-Aldrich syndrome protein, nor proline rich tyrosine kinase-2 to the target cell contact site. In addition, TIL do not flux calcium nor demonstrate proximal tyrosine kinase activity, deficiencies likely to underlie failure to fully activate the lytic machinery. Confocal microscopy and fluorescence resonance energy transfer analyses demonstrate that TIL are triggered by conjugate formation in that the TCR, p56lck, CD3ζ, LFA-1, lipid rafts, ZAP70, and linker for activation of T cells localize at the TIL:tumor cell contact site, and CD43 and CD45 are excluded. However, proximal TCR signaling is blocked upon conjugate formation because the inhibitory motif of p56lck is rapidly phosphorylated (Y505) and COOH-terminal Src kinase is recruited to the contact site, while Src homology 2 domain-containing protein phosphatase 2 is cytoplasmic. Our data support a novel mechanism explaining how tumor-induced inactivation of proximal TCR signaling regulates lytic function of antitumor T cells.

Effector-phase tolerance: another mechanism of how cancer escapes antitumor immune response.

Growth of cancer in rodent models and in patients elicits immune responses directed toward various antigens expressed by the transformed cell. Clearly though, as most tumors grow, unmanipulated antitumor immune responses are incapable of eliminating cancer. Over the past ∼15 years, antitumor immunoglobulin and T cells have been used to identify tumor antigens, which in turn, have served as the basis for therapeutic vaccine trials [ 1 , 2 ]. However, experimental cancer vaccines, although in some patients result in elimination of large tumor burdens, have a low frequency of long‐term cancer remission in most patients, ca. <5% [ 2 ]. Therefore, as tumors express antigens that distinguish themselves from nontransformed cells in immunological terms (i.e., elicit immune responses to growth of primary tumor and can target tumor cells in vivo), and tumor vaccines prime unsuccessful antitumor immune responses in patients, it is likely that growth of cancer induces immune tolerance to tumor cells. Although there are several types of T cell tolerance, mature, antigen‐specific CD8+ T cells isolated from tumors are lytic‐defective, implying that the tumor microenvironment inactivates the antitumor effector phase. The nature of the functional local tolerance to antitumor immune response is the subject of this review.

Myeloid-derived suppressor cells and anti-tumor T cells: a complex relationship.

Myeloid-Derived Suppressor Cells (MDSC) are immature myeloid cells that are potent inhibitors of immune cell function and which accumulate under conditions of inflammation, especially cancer. MDSC are suggested to promote the growth of cancer by both enhancement of tumor angiogenesis and metastasis and also inhibition of antitumor immune responses. The presence of deficient and/or defective antitumor adaptive and innate immune responses, coincident with accumulation of MDSC in lymphoid organs and tumor parenchyma, supports the notion of a causal relationship. The potent ability of MDSC to inhibit several components and phases of immune response highlights the likelihood that targeting the inhibitory functions of MDSC may maximize the therapeutic potential of antitumor immunotherapy. In order to guide the rational development of immunotherapeutic strategies that incorporate inhibition of MDSC activity and enzymatic functions, thorough understanding of the role of MDSC in antitumor immune responses is required. In this manuscript we review the multifaceted inhibitory functions of MDSC and consider the role of MDSC-induced inhibition of antitumor T cell effector phase. Support for this research is from NIH R01 CA108573.

Tumor-Induced Disruption of Proximal TCR-Mediated Signal Transduction in Tumor-Infiltrating CD8+ Lymphocytes Inactivates Antitumor Effector Phase

The presence in cancer tissue of Ag-specific, activated tumor infiltrating CD8+ T cells proves that tumors express Ags capable of eliciting immune response. Therefore, in general, tumor escape from immune-mediated clearance is not attributable to immunological ignorance. However, tumor-infiltrating lymphocytes are defective in effector phase function, demonstrating tumor-induced immune suppression that likely underlies tumor escape. Since exocytosis of lytic granules is dependent upon TCR-mediated signal transduction, it is a reasonable contention that tumors may induce defective signal transduction in tumor infiltrating T cells. In this review, we consider the biochemical basis for antitumor T cell dysfunction, focusing on the role of inhibitory signaling receptors in restricting TCR-mediated signaling in tumor-infiltrating lymphocytes.

Protein Kinase Cδ Regulates Antigen Receptor-Induced Lytic Granule Polarization in Mouse CD8+ CTL

Lytic granule exocytosis is the major pathway used by CD8+ CTL to kill virally infected and tumor cells. Despite the obvious importance of this pathway in adaptive T cell immunity, the molecular identity of enzymes involved in the regulation of this process is poorly characterized. One signal known to be critical for the regulation of granule exocytosis-mediated cytotoxicity in CD8+ T cells is Ag receptor-induced activation of protein kinase C (PKC). However, it is not known which step of the process is regulated by PKC. In addition, it has not been determined to date which of the PKC family members is required for the regulation of lytic granule exocytosis. By combination of pharmacological inhibitors and use of mice with targeted gene deletions, we show that PKCδ is required for granule exocytosis-mediated lytic function in mouse CD8+ T cells. Our studies demonstrate that PKCδ is required for lytic granule exocytosis, but is dispensable for activation, cytokine production, and expression of cytolytic molecules in response to TCR stimulation. Importantly, defective lytic function in PKCδ-deficient cytotoxic lymphocytes is reversed by ectopic expression of PKCδ. Finally, we show that PKCδ is not involved in target cell-induced reorientation of the microtubule-organizing center, but is required for the subsequent exocytosis step, i.e., lytic granule polarization. Thus, our studies identify PKCδ as a novel and selective regulator of Ag receptor-induced lytic granule polarization in mouse CD8+ T cells.

Defective Adhesion in Tumor Infiltrating CD8+ T Cells

CD8+ tumor-infiltrating lymphocytes (TIL) are defective in cytolysis due to tumor-induced inhibition of proximal TCR-mediated signaling, a defect that is relieved upon purification and brief culture. We show in this study that frequency of conjugation in vitro of nonlytic TIL with tumor cells is low in comparison with their lytic counterparts, and the strength of interaction and duration of conjugation are also reduced. Previous reports show that p56lck activation is required for TCR-initiated LFA-1 avidity up-regulation, raising the question: is low LFA-1 avidity the basis of reduced TIL conjugation frequency? When stimulated with phorbol ester, nonlytic TIL bind purified ICAM-1 equivalently as lytic TIL, suggesting that LFA-1 can be activated if proximal TCR signaling is bypassed. However, when treated with phorbol ester, the conjugation frequency of nonlytic TIL does not increase. CD2 and CD8 also mediate T cell adhesion to cognate target cells and are both expressed at lower levels in nonlytic TIL in addition to being excluded from the immune synapse formed upon conjugation. Collectively, these results imply that adhesion defects in nonlytic TIL result from a combination of decreased cell surface levels of adhesion molecules, deficient LFA-1 activation, and the failure to recruit essential adhesion receptors to the membrane contact site formed with cognate target cells.

Protocadherin-18 Is a Novel Differentiation Marker and an Inhibitory Signaling Receptor for CD8+ Effector Memory T Cells

CD8+ tumor infiltrating T cells (TIL) lack effector-phase functions due to defective proximal TCR-mediated signaling previously shown to result from inactivation of p56lck kinase. We identify a novel interacting partner for p56lck in nonlytic TIL, Protocadherin-18 (‘pcdh18’), and show that pcdh18 is transcribed upon in vitro or in vivo activation of all CD8+ central memory T cells (CD44+CD62LhiCD127+) coincident with conversion into effector memory cells (CD44+CD62LloCD127+). Expression of pcdh18 in primary CD8+ effector cells induces the phenotype of nonlytic TIL: defective proximal TCR signaling, cytokine secretion, and cytolysis, and enhanced AICD. pcdh18 contains a motif (centered at Y842) shared with src kinases (QGQYQP) that is required for the inhibitory phenotype. Thus, pcdh18 is a novel activation marker of CD8+ memory T cells that can function as an inhibitory signaling receptor and restrict the effector phase.