Dissociation of its opposing immunologic effects is critical for the optimization of antitumor CD8+ T-cell responses induced by interleukin 21

Evaluation of Cell-Mediated Immune Function Using the Cytotoxic T-Lymphocyte Assay

Evaluation of cell-mediated immunity (CMI) is a significant component in any assessment designed to predict the full range of potential immunotoxic risk underlying health risks. Among measures of CMI, the cytotoxic T-lymphocyte (CTL) response is recognized as perhaps the most relevant functional measure that reflects cell-mediated acquired immune defense against viral infections and cancer. The CTL response against T-dependent antigens requires the cooperation of at least three different major categories of immune cells. These include professional antigen-presenting cells (e.g., dendritic cells), CD4⁺ T helper lymphocytes, and CD8⁺ T effector lymphocytes. It is also among the few functional responses dependent on and, hence, capable of evaluating effective antigen presentation via both class I and class II molecules of the major histocompatibility complex (MHC). For this reason, the CTL assay is an excellent candidate for evaluation of potential immunotoxicity. This chapter provides an example of a mouse CTL assay against influenza virus that has been utilized for this purpose.

HLA-DP84Gly constitutively presents endogenous peptides generated by the class I antigen processing pathway

Classical antigen processing leads to the presentation of antigenic peptides derived from endogenous and exogenous sources for MHC class I and class II molecules, respectively. Here we show that, unlike other class II molecules, prevalent HLA-DP molecules with β-chains encoding Gly84 (DP^84Gly) constitutively present endogenous peptides. DP^84Gly does not bind invariant chain (Ii) via the class II-associated invariant chain peptide (CLIP) region, nor does it present CLIP. However, Ii does facilitate the transport of DP^84Gly from the endoplasmic reticulum (ER) to the endosomal/lysosomal pathway by transiently binding DP^84Gly via a non-CLIP region(s) in a pH-sensitive manner. Accordingly, like class I, DP^84Gly constitutively presents endogenous peptides processed by the proteasome and transported to the ER by the transporter associated with antigen processing (TAP). Therefore, DP^84Gly, found only in common chimpanzees and humans, uniquely uses both class I and II antigen-processing pathways to present peptides derived from intracellular and extracellular sources.

CD4(+) and CD8(+) TCRβ repertoires possess different potentials to generate extraordinarily high-avidity T cells

Recent high throughput sequencing analysis has revealed that the TCRβ repertoire is largely different between CD8(+) and CD4(+) T cells. Here, we show that the transduction of SIG35α, the public chain-centric HLA-A*02:01(A2)/MART127-35 TCRα hemichain, conferred A2/MART127-35 reactivity to a substantial subset of both CD8(+) and CD4(+) T cells regardless of their HLA-A2 positivity. T cells individually reconstituted with SIG35α and different A2/MART127-35 TCRβ genes isolated from CD4(+) or CD8(+) T cells exhibited a wide range of avidity. Surprisingly, approximately half of the A2/MART127-35 TCRs derived from CD4(+) T cells, but none from CD8(+) T cells, were stained by A2/MART127-35 monomer and possessed broader cross-reactivity. Our results suggest that the differences in the primary structure of peripheral CD4(+) and CD8(+) TCRβ repertoire indeed result in the differences in their ability to form extraordinarily high avidity T cells which would otherwise have been deleted by central tolerance.

Specific roles of each TCR hemichain in generating functional chain-centric TCR

TCRα- and β-chains cooperatively recognize peptide-MHC complexes. It has been shown that a "chain-centric" TCR hemichain can, by itself, dictate MHC-restricted Ag specificity without requiring major contributions from the paired TCR counterchain. Little is known, however, regarding the relative contributions and roles of chain-centric and its counter, non-chain-centric, hemichains in determining T cell avidity. We comprehensively analyzed a thymically unselected T cell repertoire generated by transducing the α-chain-centric HLA-A*02:01(A2)/MART127-35 TCRα, clone SIG35α, into A2-matched and unmatched postthymic T cells. Regardless of their HLA-A2 positivity, a substantial subset of peripheral T cells transduced with SIG35α gained reactivity for A2/MART127-35. Although the generated A2/MART127-35-specific T cells used various TRBV genes, TRBV27 predominated with >10(2) highly diverse and unique clonotypic CDR3β sequences. T cells individually reconstituted with various A2/MART127-35 TRBV27 TCRβ genes along with SIG35α possessed a wide range (>2 log orders) of avidity. Approximately half possessed avidity higher than T cells expressing clone DMF5, a naturally occurring A2/MART127-35 TCR with one of the highest affinities. Importantly, similar findings were recapitulated with other self-Ags. Our results indicate that, although a chain-centric TCR hemichain determines Ag specificity, the paired counterchain can regulate avidity over a broad range (>2 log orders) without compromising Ag specificity. TCR chain centricity can be exploited to generate a thymically unselected Ag-specific T cell repertoire, which can be used to isolate high-avidity antitumor T cells and their uniquely encoded TCRs rarely found in the periphery because of tolerance.

Early exposure to interleukin-21 limits rapidly generated anti-Epstein-Barr virus T-cell line differentiation

BACKGROUND AIMS

The adoptive transfer of ex vivo-expanded Epstein-Barr virus (EBV)-specific T-cell lines is an attractive strategy to treat EBV-related neoplasms. Current evidence suggests that for adoptive immunotherapy in general, clinical responses are superior if the transferred cells have not reached a late or terminal effector differentiation phenotype before infusion. The cytokine interleukin (IL)-21 has shown great promise at limiting late T-cell differentiation in vitro, but this remains to be demonstrated in anti-viral T-cell lines.

METHODS

We adapted a clinically validated protocol to rapidly generate EBV-specific T-cell lines in 12 to 14 days and tested whether the addition of IL-21 at the initiation of the culture would affect T-cell expansion and differentiation.

RESULTS

We generated clinical-scale EBV-restricted T-cell line expansion with balanced T-cell subset ratios. The addition of IL-21 at the beginning of the culture decreased both T-cell expansion and effector memory T-cell accumulation, with a relative increase in less-differentiated T cells. Within CD4 T-cell subsets, exogenous IL-21 was notably associated with the cell surface expression of CD27 and high KLF2 transcript levels, further arguing for a role of IL-21 in the control of late T-cell differentiation.

CONCLUSIONS

Our results show that IL-21 has profound effects on T-cell differentiation in a rapid T-cell line generation protocol and as such should be further explored as a novel approach to program anti-viral T cells with features associated with early differentiation and optimal therapeutic efficacy.

Human cell-based artificial antigen-presenting cells for cancer immunotherapy

Adoptive T-cell therapy, where anti-tumor T cells are first prepared in vitro, is attractive since it facilitates the delivery of essential signals to selected subsets of anti-tumor T cells without unfavorable immunoregulatory issues that exist in tumor-bearing hosts. Recent clinical trials have demonstrated that anti-tumor adoptive T-cell therapy, i.e. infusion of tumor-specific T cells, can induce clinically relevant and sustained responses in patients with advanced cancer. The goal of adoptive cell therapy is to establish anti-tumor immunologic memory, which can result in life-long rejection of tumor cells in patients. To achieve this goal, during the process of in vitro expansion, T-cell grafts used in adoptive T-cell therapy must be appropriately educated and equipped with the capacity to accomplish multiple, essential tasks. Adoptively transferred T cells must be endowed, prior to infusion, with the ability to efficiently engraft, expand, persist, and traffic to tumor in vivo. As a strategy to consistently generate T-cell grafts with these capabilities, artificial antigen-presenting cells have been developed to deliver the proper signals necessary to T cells to enable optimal adoptive cell therapy.

IL-21 in cancer immunotherapy: At the right place at the right time

Interleukin-21 (IL-21) has been described as a potent stimulator of antitumor T-cell immunity, but also of autoimmune reactions and oncogenesis. Antigen presenting cells genetically modified to release IL-21 allow for the expansion of tumor-specific T cells exhibiting favorable effector and growth characteristics and a minimal risk of detrimental side effects.

Membrane-bound IL-21 promotes sustained ex vivo proliferation of human natural killer cells

NK cells have therapeutic potential for a wide variety of human malignancies. However, because NK cells expand poorly in vitro, have limited life spans in vivo, and represent a small fraction of peripheral white blood cells, obtaining sufficient cell numbers is the major obstacle for NK-cell immunotherapy. Genetically-engineered artificial antigen-presenting cells (aAPCs) expressing membrane-bound IL-15 (mbIL15) have been used to propagate clinical-grade NK cells for human trials of adoptive immunotherapy, but ex vivo proliferation has been limited by telomere shortening. We developed K562-based aAPCs with membrane-bound IL-21 (mbIL21) and assessed their ability to support human NK-cell proliferation. In contrast to mbIL15, mbIL21-expressing aAPCs promoted log-phase NK cell expansion without evidence of senescence for up to 6 weeks of culture. By day 21, parallel expansion of NK cells from 22 donors demonstrated a mean 47,967-fold expansion (median 31,747) when co-cultured with aAPCs expressing mbIL21 compared to 825-fold expansion (median 325) with mbIL15. Despite the significant increase in proliferation, mbIL21-expanded NK cells also showed a significant increase in telomere length compared to freshly obtained NK cells, suggesting a possible mechanism for their sustained proliferation. NK cells expanded with mbIL21 were similar in phenotype and cytotoxicity to those expanded with mbIL15, with retained donor KIR repertoires and high expression of NCRs, CD16, and NKG2D, but had superior cytokine secretion. The mbIL21-expanded NK cells showed increased transcription of the activating receptor CD160, but otherwise had remarkably similar mRNA expression profiles of the 96 genes assessed. mbIL21-expanded NK cells had significant cytotoxicity against all tumor cell lines tested, retained responsiveness to inhibitory KIR ligands, and demonstrated enhanced killing via antibody-dependent cell cytotoxicity. Thus, aAPCs expressing mbIL21 promote improved proliferation of human NK cells with longer telomeres and less senescence, supporting their clinical use in propagating NK cells for adoptive immunotherapy.

IL-21 can supplement suboptimal Lck-independent MAPK activation in a STAT-3-dependent manner in human CD8(+) T cells

Although both MHC class II/CD8α double-knockout and CD8β null mice show a defect in the development of MHC class I-restricted CD8(+) T cells in the thymus, they possess low numbers of high-avidity peripheral CTL with limited clonality and are able to contain acute and chronic infections. These in vivo data suggest that the CD8 coreceptor is not absolutely necessary for the generation of Ag-specific CTL. Lack of CD8 association causes partial TCR signaling because of the absence of CD8/Lck recruitment to the proximity of the MHC/TCR complex, resulting in suboptimal MAPK activation. Therefore, there should exist a signaling mechanism that can supplement partial TCR activation caused by the lack of CD8 association. In this human study, we have shown that CD8-independent stimulation of Ag-specific CTL previously primed in the presence of CD8 coligation, either in vivo or in vitro, induced severely impaired in vitro proliferation. When naive CD8(+) T cells were primed in the absence of CD8 binding and subsequently restimulated in the presence of CD8 coligation, the proliferation of Ag-specific CTL was also severely hampered. However, when CD8-independent T cell priming and restimulation were supplemented with IL-21, Ag-specific CD8(+) CTL expanded in two of six individuals tested. We found that IL-21 rescued partial MAPK activation in a STAT3- but not STAT1-dependent manner. These results suggest that CD8 coligation is critical for the expansion of postthymic peripheral Ag-specific CTL in humans. However, STAT3-mediated IL-21 signaling can supplement partial TCR signaling caused by the lack of CD8 association.

An integrated disease/pharmacokinetic/pharmacodynamic model suggests improved interleukin-21 regimens validated prospectively for mouse solid cancers

Interleukin (IL)-21 is an attractive antitumor agent with potent immunomodulatory functions. Yet thus far, the cytokine has yielded only partial responses in solid cancer patients, and conditions for beneficial IL-21 immunotherapy remain elusive. The current work aims to identify clinically-relevant IL-21 regimens with enhanced efficacy, based on mathematical modeling of long-term antitumor responses. For this purpose, pharmacokinetic (PK) and pharmacodynamic (PD) data were acquired from a preclinical study applying systemic IL-21 therapy in murine solid cancers. We developed an integrated disease/PK/PD model for the IL-21 anticancer response, and calibrated it using selected “training” data. The accuracy of the model was verified retrospectively under diverse IL-21 treatment settings, by comparing its predictions to independent “validation” data in melanoma and renal cell carcinoma-challenged mice (R²>0.90). Simulations of the verified model surfaced important therapeutic insights: (1) Fractionating the standard daily regimen (50 µg/dose) into a twice daily schedule (25 µg/dose) is advantageous, yielding a significantly lower tumor mass (45% decrease); (2) A low-dose (12 µg/day) regimen exerts a response similar to that obtained under the 50 µg/day treatment, suggestive of an equally efficacious dose with potentially reduced toxicity. Subsequent experiments in melanoma-bearing mice corroborated both of these predictions with high precision (R²>0.89), thus validating the model also prospectively in vivo. Thus, the confirmed PK/PD model rationalizes IL-21 therapy, and pinpoints improved clinically-feasible treatment schedules. Our analysis demonstrates the value of employing mathematical modeling and in silico-guided design of solid tumor immunotherapy in the clinic.

Among the many potential drugs explored within the scope of cancer immunotherapy are selected cytokines which possess promising immune-boosting properties. Yet, the natural involvement of these proteins in multiple, often contradicting biological processes can complicate their use in the clinic. The cytokine interleukin (IL)-21 is no exception: while its strength as an anticancer agent has been established in several animal studies, response rates in melanoma and renal cell carcinoma patients remain low. To help guide the design of effective IL-21 therapy, we have developed a mathematical model that bridges between the complex biology of IL-21 and its optimal clinical use. Our model integrates data from preclinical studies under diverse IL-21 treatment settings, and was validated by extensive experiments in tumor-bearing mice. Model simulations predicted that beneficial, clinically practical IL-21 therapy should be composed of low-dose schedules, and/or schedules in which several partial doses are administered rather than a single complete dose. These findings were subsequently confirmed in mice with melanoma. Thus, future testing of these strategies in solid cancer patients can be a promising starting point for improving IL-21 therapy. Our model can thus provide a computational platform for rationalizing IL-21 regimens and streamlining its clinical development.

Induction of HLA-DP4-restricted anti-survivin Th1 and Th2 responses using an artificial antigen-presenting cell

PURPOSE

In previous cancer vaccine clinical trials targeting survivin, induction of specific CD8(+) T-cell responses did not consistently lead to clinical responses. Considering the critical role of CD4(+) T-cell help in generating antitumor immunity, integration of anti-survivin CD4(+) T-cell responses may enhance the efficacy of anti-survivin cancer immunotherapy. Human leukocyte antigen (HLA)-DP4 is emerging as an attractive MHC target allele of CD4(+) T cell-mediated immunotherapy, because it is one of the most frequent HLA alleles in many ethnic groups. In this article, we aimed to elucidate DP4-restricted CD4(+) T-cell responses against survivin in cancer patients.

EXPERIMENTAL DESIGN

We generated a human cell-based artificial antigen-presenting cell (aAPC) expressing HLA-DP4, CD80, and CD83 and induced DP4-restricted antigen-specific CD4(+) T cells. The number, phenotype, effector function, and in vitro longevity of generated CD4(+) T cells were determined.

RESULTS

We first determined previously unknown DP4-restricted CD4(+) T-cell epitopes derived from cytomegalovirus pp65, to which sustained Th1-biased recall responses were induced in vitro by using DP4-aAPC. In contrast, DP4-aAPC induced in vitro both Th1 and Th2 long-lived anti-survivin CD4(+) T cells from cancer patients. Both survivin-specific Th1 and Th2 cells were able to recognize survivin-expressing tumors in a DP4-restricted manner. Neither survivin-specific interleukin 10 secreting Tr1 cells nor Th17 cells were induced by DP4-aAPC.

CONCLUSIONS

DP4-restricted anti-survivin Th1 and Th2 immunity with sufficient functional avidity can be induced from cancer patients. The development of strategies to concurrently induce both CD4(+) and CD8(+) T-cell responses against survivin is warranted for optimal anti-survivin cancer immunotherapy.

IL-21-treated naive CD45RA+ CD8+ T cells represent a reliable source for producing leukemia-reactive cytotoxic T lymphocytes with high proliferative potential and early differentiation phenotype

Clinical tumor remissions after adoptive T-cell therapy are frequently not durable due to limited survival and homing of transfused tumor-reactive T cells, what can be mainly attributed to the long-term culture necessary for in vitro expansion. Here, we introduce an approach allowing the reliable in vitro generation of leukemia-reactive cytotoxic T lymphocytes (CTLs) from naive CD8+ T cells of healthy donors, leading to high cell numbers within a relatively short culture period. The protocol includes the stimulation of purified CD45RA+ CD8+ T cells with primary acute myeloid leukemia blasts of patient origin in HLA-class I-matched allogeneic mixed lymphocyte-leukemia cultures. The procedure allowed the isolation of a large diversity of HLA-A/-B/-C-restricted leukemia-reactive CTL clones and oligoclonal lines. CTLs showed reactivity to either leukemia blasts exclusively, or to leukemia blasts as well as patient-derived B lymphoblastoid-cell lines (LCLs). In contrast, LCLs of donor origin were not lysed. This reactivity pattern suggested that CTLs recognized leukemia-associated antigens or hematopoietic minor histocompatibility antigens. Consistent with this hypothesis, most CTLs did not react with patient-derived fibroblasts. The efficiency of the protocol could be further increased by addition of interleukin-21 during primary in vitro stimulation. Most importantly, leukemia-reactive CTLs retained the expression of early T-cell differentiation markers CD27, CD28, CD62L and CD127 for several weeks during culture. The effective in vitro expansion of leukemia-reactive CD8+ CTLs from naive CD45RA+ precursors of healthy donors can accelerate the molecular definition of candidate leukemia antigens and might be of potential use for the development of adoptive CTL therapy in leukemia.

A panel of human cell-based artificial APC enables the expansion of long-lived antigen-specific CD4+ T cells restricted by prevalent HLA-DR alleles

Many preclinical experiments have attested to the critical role of CD4(+) T cell help in CD8(+) cytotoxic T lymphocyte (CTL)-mediated immunity. Recent clinical trials have demonstrated that reinfusion of CD4(+) T cells can induce responses in infectious diseases and cancer. However, few standardized and versatile systems exist to expand antigen-specific CD4(+) T(h) for clinical use. K562 is a human erythroleukemic cell line, which lacks expression of HLA class I and class II, invariant chain and HLA-DM but expresses adhesion molecules such as intercellular adhesion molecule-1 and leukocyte function-associated antigen-3. With this unique immunologic phenotype, K562 has been tested in clinical trials of cancer immunotherapy. Previously, we created a K562-based artificial antigen-presenting cell (aAPC) that generates ex vivo long-lived HLA-A2-restricted CD8(+) CTL with a central/effector memory phenotype armed with potent effector function. We successfully generated a clinical version of this aAPC and conducted a clinical trial where large numbers of anti-tumor CTL are reinfused to cancer patients. In this article, we shifted focus to CD4(+) T cells and developed a panel of novel K562-derived aAPC, where each expresses a different single HLA-DR allele, invariant chain, HLA-DM, CD80, CD83 and CD64; takes up soluble protein by endocytosis and processes and presents CD4(+) T-cell peptides. Using this aAPC, we were able to determine novel DR-restricted CD4(+) T-cell epitopes and expand long-lived CD4(+) T-cells specific for multiple antigens without growing bystander Foxp3(+) regulatory T cells. Our results suggest that K562-based aAPC may serve as a translatable platform to generate both antigen-specific CD8(+) CTL and CD4(+) T(h).

The cytotoxic T lymphocyte assay for evaluating cell-mediated immune function

Evaluation of cell-mediated immunity (CMI) is a significant component in any assessment designed to predict the full range of potential immunotoxic risk underlying health risks. Among measures of CMI, the cytotoxic T Lymphocyte (CTL) response is recognized as perhaps the most relevant functional measure that reflects cell-mediated acquired immune defense against viral infections and cancer. The CTL response against T-dependent antigens requires the cooperation of at least three different major categories of immune cells. These include professional antigen presenting cells (e.g., dendritic cells), CD4(+) T helper lymphocytes, and CD8(+) T effector lymphocytes. It is also among the few functional responses dependent on and, hence, capable of evaluating effective antigen presentation via both class I and class II molecules of the major histocompatibility complex (MHC). For this reason the CTL assay is an excellent candidate for evaluation of potential immunotoxicity. This chapter provides an example of a mouse CTL assay against influenza virus that has been utilized for this purpose.

Interleukin-21 restrains tumor growth and induces a substantial increase in the number of circulating tumor-specific T cells in a murine model of malignant melanoma

New strategies of immunotherapy are currently being evaluated, and the combination of chemo- and immunotherapy has shown promising results. The cytokine interleukin-21 (IL-21) is known to enhance immune function, and in this study we have investigated its ability to boost the efficacy of chemoimmunotherapy-cyclophosphamide and adoptive cell transfer (ACT)-in the B16-OVA/OT-I murine model of malignant melanoma. Subcutaneous B16-OVA tumors were established in C57BL/6J mice 8 days before adoptive transfer of tumor-specific OT-I T cells. In addition to cyclophosphamide and ACT, one group of mice received daily injections of murine IL-21 (mIL-21). Mice treated with mIL-21 had more tumor-specific T cells in the circulation 4 and 7 days following ACT (P=0.004 and P=0.002, respectively). Importantly, mIL-21 and ACT controlled tumor growth instantly and more effectively than ACT alone (P=0.001, day 4)-an effect that persisted up to 5 days after the last mIL-21 injection. We conclude that mIL-21 enhances chemoimmunotherapy: it amplifies the number of tumor-specific T cells in the circulation and also stunts early tumor growth.

Genetic modification of T cells with IL-21 enhances antigen presentation and generation of central memory tumor-specific cytotoxic T-lymphocytes

An optimized antigen-presenting cell for tumor immunotherapy should produce a robust antigen specific cytotoxic T lymphocytes (CTL) response to tumor-associated antigens, which can persist in vivo and expand on antigen reencounter. Interleukin (IL)-21 synergizes with other gamma-chain cytokines to enhance the frequency and cytotoxicity of antigen-specific CTL. As T cells themselves may serve as effective antigen-presenting cells (T antigen-presenting cells; TAPC) and may be useful in vivo as cellular vaccines, we examined whether CD8(+) T cells genetically modified to produce IL-21 could induce immune responses to tumor associated antigen peptides in healthy human leukocyte antigen-A2(+) donors. We found that IL-21 modified TAPC enhanced both the proliferation and survival of MART-1 specific CD8(+) T cells, which were enriched by >8-fold over cultures with control nontransgenic TAPC. MART-1-specific CTL produced interferon-gamma in response to cognate peptide antigen and killed primary tumor cells expressing MART-1 in a major histocompatibility complex restricted manner. IL-21 modified TAPC similarly enhanced generation of functional CTL against melanoma antigen gp100 and the B-cell chronic lymphocytic leukemia associated RHAMM antigen. Antigen-specific CTL generated using IL-21 gene-modified TAPC had a central memory phenotype characterized by CD45RA(-), CD44(high), CD27(high), CD28(high), CD62L(high), and IL-7 receptor-alpha(high), contrasting with the terminal effector phenotype of CTL generated in the absence of IL-21. Thus, TAPC stimulation in the presences of IL-21 enhances proliferation of tumor antigen-specific T cells and favors induction of a central memory phenotype, which may improve proliferation, survival, and efficacy of T-cell based therapies for the treatment of cancer.