The spleen is the main filter for blood-borne pathogens and antigens, as well as a key organ for iron metabolism and erythrocyte homeostasis. However, immune and hematopoietic functions have been recently unveiled for the mouse spleen, suggesting additional roles for this secondary lymphoid organ. Here we discuss the integration of the spleen in the regulation of immune responses locally and in the whole body and present the relevance of findings for our understanding of inflammatory and degenerative diseases and their treatments. We also consider whether equivalent activities in humans are known, as well as initial therapeutic attempts to target the spleen for modulating innate and adaptive immunity.
One obstacle in eliciting potent anti-tumor immune responses is the induction of tolerance to tumor antigens. TCRlo mice bearing a TCR transgene specific for the melanoma antigen Tyrosinase-related protein-2 (TRP-2, Dct) harbor T cells that maintain tumor antigen responsiveness but lack the ability to control melanoma outgrowth. We used this model to determine whether higher avidity T cells could control tumor growth without becoming tolerized. As part of the current study, we developed a second TRP-2-specific TCR transgenic mouse line (TCRhi) that bears higher avidity T cells and spontaneously developed autoimmune depigmentation. In contrast to TCRlo T cells, which were ignorant of tumor-derived antigen, TCRhi T cells initially delayed subcutaneous B16 melanoma tumor growth. However, persistence in the tumor microenvironment resulted in reduced IFN-γ production and CD107a (Lamp1) mobilization, hallmarks of T cell tolerization. IFN-γ expression by TCRhi T cells was critical for up-regulation of MHC-I on tumor cells and control of tumor growth. Blockade of PD-1 signals prevented T cell tolerization and restored tumor immunity. Depletion of tumor-associated dendritic cells (TADCs) reduced tolerization of TCRhi T cells and enhanced their antitumor activity. In addition, TADCs tolerized TCRhi T cells but not TCRlo T cells in vitro. Our findings demonstrate that T cell avidity is a critical determinant of not only tumor control but also susceptibility to tolerization in the tumor microenvironment. For this reason, care should be exercised when considering T cell avidity in designing cancer immunotherapeutics.
melanoma; T cells; avidity; adoptive immunotherapy; tolerance
Hypoxia selectively up-regulates PD-L1 on myeloid-derived suppressor cells via HIF-1a, thus affecting T cell activation.
Tumor-infiltrating myeloid cells such as myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs) form an important component of the hypoxic tumor microenvironment. Here, we investigated the influence of hypoxia on immune checkpoint receptors (programmed death [PD]-1 and CTLA-4) and their respective ligands (PD-1 ligand 1 [PD-L1], PD-L2, CD80, and CD86) on MDSCs. We demonstrate that MDSCs at the tumor site show a differential expression of PD-L1 as compared with MDSCs from peripheral lymphoid organ (spleen). Hypoxia caused a rapid, dramatic, and selective up-regulation of PD-L1 on splenic MDSCs in tumor-bearing mice. This was not limited to MDSCs, as hypoxia also significantly increased the expression of PD-L1 on macrophages, dendritic cells, and tumor cells. Furthermore, PD-L1 up-regulation under hypoxia was dependent on hypoxia-inducible factor-1α (HIF-1α) but not HIF-2α. Chromatin immunoprecipitation and luciferase reporter assay revealed direct binding of HIF-1α to a transcriptionally active hypoxia-response element (HRE) in the PD-L1 proximal promoter. Blockade of PD-L1 under hypoxia enhanced MDSC-mediated T cell activation and was accompanied by the down-regulation of MDSCs IL-6 and IL-10. Finally, neutralizing antibodies against IL-10 under hypoxia significantly abrogated the suppressive activity of MDSCs. Simultaneous blockade of PD-L1 along with inhibition of HIF-1α may thus represent a novel approach for cancer immunotherapy.
The present study employed mass sequencing of small RNA libraries to identify the repertoire of small noncoding RNAs expressed in normal CD4+ T cells compared to cells transformed with human T-cell leukemia virus type 1 (HTLV-1), the causative agent of adult T-cell leukemia/lymphoma (ATLL). The results revealed distinct patterns of microRNA expression in HTLV-1-infected CD4+ T-cell lines with respect to their normal counterparts. In addition, a search for virus-encoded microRNAs yielded 2 sequences that originated from the plus strand of the HTLV-1 genome. Several sequences derived from tRNAs were expressed at substantial levels in both uninfected and infected cells. One of the most abundant tRNA fragments (tRF-3019) was derived from the 3′ end of tRNA-proline. tRF-3019 exhibited perfect sequence complementarity to the primer binding site of HTLV-1. The results of an in vitro reverse transcriptase assay verified that tRF-3019 was capable of priming HTLV-1 reverse transcriptase. Both tRNA-proline and tRF-3019 were detected in virus particles isolated from HTLV-1-infected cells. These findings suggest that tRF-3019 may play an important role in priming HTLV-1 reverse transcription and could thus represent a novel target to control HTLV-1 infection.
IMPORTANCE Small noncoding RNAs, a growing family of regulatory RNAs that includes microRNAs and tRNA fragments, have recently emerged as key players in many biological processes, including viral infection and cancer. In the present study, we employed mass sequencing to identify the repertoire of small noncoding RNAs in normal T cells compared to T cells transformed with human T-cell leukemia virus type 1 (HTLV-1), a retrovirus that causes adult T-cell leukemia/lymphoma. The results revealed a distinct pattern of microRNA expression in HTLV-1-infected cells and a tRNA fragment (tRF-3019) that was packaged into virions and capable of priming HTLV-1 reverse transcription, a key event in the retroviral life cycle. These findings indicate tRF-3019 could represent a novel target for therapies aimed at controlling HTLV-1 infection.
Despite extensive ex vivo investigation, the spatiotemporal organization of immune cells interacting with virus-infected cells in tissues remains uncertain. To address this, we used intravital multiphoton microscopy to visualize immune cell interactions with virus-infected cells following epicutaneous vaccinia virus (VV) infection of mice. VV infects keratinocytes in epidermal foci, and numerous migratory dermal inflammatory monocytes outlying the foci. We observed Ly6G+ innate immune cells infiltrating and controlling foci, while CD8+ T cells remained on the periphery killing infected monocytes. Most antigen-specific CD8+ T cells in the skin did not interact with virus-infected cells. Blocking the generation of reactive nitrogen species relocated CD8+ T cells into foci, modestly reducing viral titers. Depletion of Ly6G+ and CD8+ cells dramatically increased viral titers, consistent with their synergistic but spatially segregated viral clearance activities. These findings highlight previously unappreciated differences in the anatomic specialization of antiviral immune cell subsets.
Under many inflammatory contexts, such as tumor progression, systemic and peripheral immune response is tailored by reactive nitrogen species (RNS)-dependent post-translational modifications, suggesting a biological function for these chemical alterations. RNS modify both soluble factors and receptors essential to induce and maintain a tumor-specific immune response, creating a “chemical barrier” that impairs effector T cell infiltration and functionality in tumor microenvironment and supports the escape phase of cancer. RNS generation during tumor growth mainly depends on nitric oxide production by both tumor cells and tumor-infiltrating myeloid cells that constitutively activate essential metabolic pathways of l-arginine catabolism. This review provides an overview of the potential immunological and biological role of RNS-induced modifications and addresses new approaches targeting RNS either in search of novel biomarkers or to improve anti-cancer treatment.
RNS; nitrotyrosine; cancer; microenvironment; immune escape
Myeloid cells are the most abundant nucleated hematopoietic cells in the human body and are a collection of distinct cell populations with many diverse functions. The three groups of terminally differentiated myeloid cells — macrophages, dendritic cells and granulocytes — are essential for the normal function of both the innate and adaptive immune systems. Mounting evidence indicates that the tumour microenvironment alters myeloid cells and can convert them into potent immune suppressive cells. Here, we consider myeloid cells as an intricately connected, complex, single system and we focus on how tumours manipulate the myeloid system to evade the host immune response.
Acute myeloid leukemia (AML) is a genetically heterogeneous clonal disorder characterized by two molecularly distinct self-renewing leukemic stem cell (LSC) populations most closely related to normal progenitors and organized as a hierarchy. A requirement for WNT/β-catenin signaling in the pathogenesis of AML has recently been suggested by a mouse model. However, its relationship to a specific molecular function promoting retention of self-renewing leukemia-initiating cells (LICs) in human remains elusive. To identify transcriptional programs involved in the maintenance of a self-renewing state in LICs, we performed the expression profiling in normal (n = 10) and leukemic (n = 33) human long-term reconstituting AC133+ cells, which represent an expanded cell population in most AML patients. This study reveals the ligand-dependent WNT pathway activation in AC133bright AML cells and shows a diffuse expression and release of WNT10B, a hematopoietic stem cell regenerative-associated molecule. The establishment of a primary AC133+ AML cell culture (A46) demonstrated that leukemia cells synthesize and secrete WNT ligands, increasing the levels of dephosphorylated β-catenin in vivo. We tested the LSC functional activity in AC133+ cells and found significant levels of engraftment upon transplantation of A46 cells into irradiated Rag2-/-γc-/- mice. Owing to the link between hematopoietic regeneration and developmental signaling, we transplanted A46 cells into developing zebrafish. This system revealed the formation of ectopic structures by activating dorsal organizer markers that act downstream of the WNT pathway. In conclusion, our findings suggest that AC133bright LSCs are promoted by misappropriating homeostatic WNT programs that control hematopoietic regeneration.
The tumor microenvironment contains a vast array of pro- and anti-inflammatory cytokines that alter myelopoiesis and lead to the maturation of immunosuppressive cells known as myeloid-derived suppressor cells (MDSCs). Incubating bone marrow (BM) precursors with a combination of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-6 (IL-6) generated a tumor-infiltrating MDSC-like population that impaired anti-tumor specific T-cell functions. This in vitro experimental approach was used to simulate MDSC maturation, and the cellular metabolic response was then monitored. A complementary experimental model that inhibited L-arginine (L-Arg) metabolizing enzymes in MSC-1 cells, an immortalized cell line derived from primary MDSCs, was used to study the metabolic events related to immunosuppression.
Exposure of BM cells to GM-CSF and IL-6 activated, within 24 h, L-Arg metabolizing enzymes which are responsible for the MDSCs immunosuppressive potential. This was accompanied by an increased uptake of L-glutamine (L-Gln) and glucose, the latter being metabolized by anaerobic glycolysis. The up-regulation of nutrient uptake lead to the accumulation of TCA cycle intermediates and lactate as well as the endogenous synthesis of L-Arg and the production of energy-rich nucleotides. Moreover, inhibition of L-Arg metabolism in MSC-1 cells down-regulated central carbon metabolism activity, including glycolysis, glutaminolysis and TCA cycle activity, and led to a deterioration of cell bioenergetic status. The simultaneous increase of cell specific concentrations of ATP and a decrease in ATP-to-ADP ratio in BM-derived MDSCs suggested cells were metabolically active during maturation. Moreover, AMP-activated protein kinase (AMPK) was activated during MDSC maturation in GM-CSF and IL-6–treated cultures, as revealed by the continuous increase of AMP-to-ATP ratios and the phosphorylation of AMPK. Likewise, AMPK activity was decreased in MSC-1 cells when L-Arg metabolizing enzymes were inhibited. Finally, inhibition of AMPK activity by the specific inhibitor Compound C (Comp-C) resulted in the inhibition of L-Arg metabolizing enzyme activity and abolished MDSCs immunosuppressive activity.
We anticipate that the inhibition of AMPK and the control of metabolic fluxes may be considered as a novel therapeutic target for the recovery of the immunosurveillance process in cancer-bearing hosts.
Myeloid-derived suppressor cells; GM-CSF; IL-6; MSC-1 cells; Central carbon metabolism; Bioenergetics
We described a novel tumor-associated immunosuppressive mechanism based on post-translational modifications of chemokines by reactive nitrogen species (RNS). To overcome tumor immunosuppressive hindrances, we designed and developed a new drug, AT38, that inhibits RNS generation at the tumor site. Combinatorial approaches with AT38 boost the effectiveness of cancer immunotherapy protocols.
CTLs; MDSCs; cancer; chemokines; immunotherapy; reactive nitrogen species
Blocking CCL2 nitration in tumors promoted CD8+ influx and reduced tumor growth and prolonged survival in mice when combined with adoptive cell therapy.
Tumor-promoted constraints negatively affect cytotoxic T lymphocyte (CTL) trafficking to the tumor core and, as a result, inhibit tumor killing. The production of reactive nitrogen species (RNS) within the tumor microenvironment has been reported in mouse and human cancers. We describe a novel RNS-dependent posttranslational modification of chemokines that has a profound impact on leukocyte recruitment to mouse and human tumors. Intratumoral RNS production induces CCL2 chemokine nitration and hinders T cell infiltration, resulting in the trapping of tumor-specific T cells in the stroma that surrounds cancer cells. Preconditioning of the tumor microenvironment with novel drugs that inhibit CCL2 modification facilitates CTL invasion of the tumor, suggesting that these drugs may be effective in cancer immunotherapy. Our results unveil an unexpected mechanism of tumor evasion and introduce new avenues for cancer immunotherapy.
Cancer creates a peculiar inflammatory environment enriched for transcription factors with a negative influence on adaptive immunity. In this issue of the JCI, Watkins and colleagues identify Foxo3 as a master regulator of the tolerogenic program in tumor-associated, plasmacytoid DCs (pDCs). Foxo3 enables pDCs to induce tolerance in tumor antigen-specific CD8+ T cells, turning them into regulatory lymphocytes capable of inhibiting nearby CD8+ T lymphocytes. Provision of tumor-specific CD4+ T helper cells interrupts this circuit by inhibiting Foxo3 expression and fully licensing the antigen-presenting ability of pDCs. These data identify a new target for therapeutic intervention and provide insight into the transcription factor interplay in myeloid cells recruited to the cancer microenvironment.
In diabetes chronic hyperinsulinemia contributes to the instability of the atherosclerotic plaque and stimulates cellular proliferation through the activation of the MAP kinases, which in turn regulate cellular proliferation. However, it is not known whether insulin itself could increase the transcription of specific genes for cellular proliferation in the endothelium. Hence, the characterization of transcriptional modifications in endothelium is an important step for a better understanding of the mechanism of insulin action and the relationship between endothelial cell dysfunction and insulin resistance.
Methodology and principal findings
The transcriptional response of endothelial cells in the 440 minutes following insulin stimulation was monitored using microarrays and compared to a control condition. About 1700 genes were selected as differentially expressed based on their treated minus control profile, thus allowing the detection of even small but systematic changes in gene expression. Genes were clustered in 7 groups according to their time expression profile and classified into 15 functional categories that can support the biological effects of insulin, based on Gene Ontology enrichment analysis. In terms of endothelial function, the most prominent processes affected were NADH dehydrogenase activity, N-terminal myristoylation domain binding, nitric-oxide synthase regulator activity and growth factor binding. Pathway-based enrichment analysis revealed “Electron Transport Chain” significantly enriched.
Results were validated on genes belonging to “Electron Transport Chain” pathway, using quantitative RT-PCR.
As far as we know, this is the first systematic study in the literature monitoring transcriptional response to insulin in endothelial cells, in a time series microarray experiment. Since chronic hyperinsulinemia contributes to the instability of the atherosclerotic plaque and stimulates cellular proliferation, some of the genes identified in the present work are potential novel candidates in diabetes complications related to endothelial dysfunction.
The development of effective anti-tumor immune responses is normally constrained by low avidity, tumor-specific cytotoxic T lymphocytes (CTLs) which are unable to eradicate the tumor. Strategies to rescue anti-tumor activity of low avidity melanoma-specific CTLs in vivo may improve immunotherapy efficacy. To boost the in vivo effectiveness of low avidity CTLs we immunized mice bearing lung melanoma metastases with artificial Antigen Presenting Cells (aAPC), made by covalently coupling pepMHC-Ig dimers and B7.1-Ig molecules to magnetic beads. aAPC treatment induced significant tumor reduction in a mouse telomerase antigen system and complete tumor eradication in a mouse TRP-2 antigen system, when low avidity CTLs specific for these antigens were adoptively transferred. In addition, in an in vivo treatment model of subcutaneous melanoma, aAPC injection also augmented the activity of adoptively transferred CTLs and significantly delayed tumor growth. In vivo tumor clearance due to aAPC administration correlated with in situ proliferation of the transferred CTL. In vitro studies showed that aAPC effectively stimulated cytokine release, enhanced CTL-mediated lysis and TCR down-regulation in low avidity CTLs. Therefore, in vivo aAPC administration represents a potentially novel approach to improve cancer immunotherapy.
artificial Antigen Presenting Cells (aAPC); melanoma; adoptive transfer; CTL affinity; costimulation
Several studies strongly suggest that DC differentiated in vitro in the presence of type I IFN acquire more potent immune stimulatory properties, compared with DC differentiated in vitro with IL-4. However, little is known about the molecular mechanisms underlying this phenomenon. To address this question, we compared the Ag-processing machinery (APM) profile in human DC grown in the presence of IFN-α (IFNDC) or IL-4 (IL-4DC). Using a panel of APM component-specific mAb in Western blot experiments, we found that IFNDC preferentially express inducible proteasome subunits (LMP2, LMP7, and MECL1) both at immature and mature stages. In contrast, immature IL-4DC co-express both constitutive (β1, β2, and β5) and inducible subunits, as shown by Western blotting analysis. In addition, immature IFNDC express higher levels of TAP1, TAP2, calnexin, calreticulin, tapasin, and HLA class I molecules than IL-4DC. The different proteasome profiles of IFNDC and IL-4DC were associated with a greater ability of IFNDC to present an immunodominant epitope that requires LMP7 expression for its processing. In general, these data show the impact of cytokines on APM component expression and hence the Ag-processing ability of DC.
Antibodies; Antigen processing; Cytokines; Human DC
Apoptotic death of CD8+ T cells can be induced by a population of inhibitory myeloid cells that are double positive for the CD11b and Gr-1 markers. These cells are responsible for the immunosuppression observed in pathologies as dissimilar as tumor growth and overwhelming infections, or after immunization with viruses. The appearance of a CD11b+/Gr-1+ population of inhibitory macrophages (i Macs) could be attributed to high levels of granulocyte-macrophage colony-stimulating factor (GM-CSF) in vivo. Deletion of i Macs in vitro or in vivo reversed the depression of CD8+ T-cell function. We isolated i Macs from the spleens of immunocompromised mice and found that these cells were positive for CD31, ER-MP20 (Ly-6C), and ER-MP58, markers characteristic of granulocyte/monocyte precursors. Importantly, although i Macs retained their inhibitory properties when cultured in vitro in standard medium, suppressive functions could be modulated by cytokine exposure. Whereas culture with the cytokine interleukin 4 (IL-4) increased i Mac inhibitory activity, these cells could be differentiated into a nonadherent population of fully mature and highly activated dendritic cells when cultured in the presence of IL-4 and GM-CSF. A common CD31+/CD11b+/Gr-1+ progenitor can thus give rise to cells capable of either activating or inhibiting the function of CD8+ T lymphocytes, depending on the cytokine milieu that prevails during antigen-presenting cell maturation.
Interleukin-10 (IL-10) has a wide range of in vivo biological activities and is a key regulatory cytokine of immune-mediated inflammation. The authors found that murine IL-10 given 12 hours after a recombinant vaccinia virus (rVV) containing the LacZ gene significantly enhanced the treatment of mice bearing 3-day-old pulmonary metastases expressing β-galactosidase. Because IL-10 has been shown to inhibit the functions of key elements of both innate and acquired immune responses, the authors hypothesized that IL-10 might act by inhibiting clearance of the rVV, thus prolonging exposure to the experimental antigen. However, evidence that IL-10 was not acting primarily through such negative regulatory mechanisms included the following: (a) IL-10 also enhanced the therapeutic effectiveness of a recombinant fowlpox virus, which cannot replicate in mammalian cells; (b) Titers of rVV in immunized mice were lower, not higher; and (c) Although IL-10 did not alter levels of anti-vaccinia antibodies or natural killer cell activity, rVV-primed mice treated with IL-10 had enhanced vaccinia-specific cytotoxic T-lymphocyte activity. Thus, IL-10 enhanced the function of a recombinant poxvirus-based anti-cancer vaccine and may represent a potential adjuvant in the vaccination against human cancers using recombinant poxvirus-based vaccines.
Interleukin-10; Recombinant vaccinia virus; Poxvirus; Murine tumor model
The recent cloning of tumor-associated antigens (TAAs) recognized by CD8 + T lymphocytes (TCD8−) has made it possible to use recombinant and synthetic forms of TAAs to generate TCD8− with anti-tumor activity. To explore new therapeutic strategies in a mouse model, we retrovirally transduced the experimental murine tumor CT26 (H-2d), with the lacZ gene encoding our model TAA, (β-galactosidase (β-gal). The transduced cell line, CT26.CL25, grew as rapidly and as lethally as the parental cell line in normal, immuno-competent animals. In an attempt to elicit TCD8+ directed against our model TAA by using purely recombinant and synthetic forms of our model TAA, we synthesized a nine-amino-acid long immunodominant peptide of (β-gal (TPH-PARIGL), corresponding to amino acid residues 876–884, which was known to be presented by the Ld major histocompatibility complex (MHC) class I molecule, and a recombinant vaccinia virus encoding the full-length β-gal protein (VJS6). Splenocytes obtained from naïve mice and co-cultured with (β-gal peptide could not be expanded in primary ex vivo cultures. However, mice immunized with VJS6, but not with a control recombinant vaccinia virus, yielded splenocytes that were capable of specifically lysing CT26.CL25 in vitro after co-culture with (β-gal peptide. Most significantly, adoptive transfer of these cells could effectively treat mice bearing 3-day-old established pulmonary metastases. These observations show that therapeutic TCD8+ directed against a model TAA could be generated by using purely recombinant and synthetic forms of this antigen. These findings point the way to a potentially useful immunotherapeutic strategy, which has been made possible by the recent cloning of immunogenic TAAs that are expressed by human malignancies.
Recombinant vaccinia virus; Peptide; Adoptive immunotherapy; T lymphocyte; MHC class I
Activation of T lymphocytes in the absence of a costimulatory signal can result in anergy or apoptotic cell death. Two molecules capable of providing a costimulatory signal, B7-1 (CD80) and B7-2 (CD86), have been shown to augment the immunogenicity of whole-tumor cell vaccines. To explore a potential role for costimulation in the design of recombinant anticancer vaccines, we used lacZ-transduced CT26 as an experimental tumor and β-galactosidase (β-gal) as the model tumor antigen. Attempts to augment the function of a recombinant vaccinia virus (rVV) expressing β-gal by admixture with rVV expressing murine B7-1 were unsuccessful. However, a double recombinant vaccinia virus engineered to express both B7-1 and the model antigen β-gal was capable of significantly reducing the number of pulmonary metastases when administered to mice bearing tumors established for 3 or 6 days. Most important, the double recombinant vaccinia virus prolonged the survival of tumor-bearing mice. These effects were antigen specific. The related costimulatory molecule B7-2 was found to have a similar, although less impressive enhancing effect on the function of a rVV expressing β-gal. Thus, the addition of B7-1 and, to a lesser extent, B7-2 to a rVV encoding a model antigen significantly enhanced the therapeutic antitumor effects of these poxvirus-based, therapeutic anticancer vaccines.
“Self” melanocyte differentiation antigens are potential targets for specific melanoma immunotherapy. Vaccination against murine tyrosinase-related protein (TRP)-1/gp75 was shown recently to cause melanoma rejection, which was accompanied by autoimmune skin depigmentation (vitiligo). To further explore the linkage between immunotherapy and autoimmunity, we studied the response to vaccination with a related antigen, TRP-2. i.m. inoculation of plasmid DNA encoding murine trp-2 elicited antigen-specific CTLs that recognized the B16 mouse melanoma and protected the mice from challenge with tumor cells. Furthermore, mice bearing established s.c. B16 melanomas rejected the tumor upon vaccination with a recombinant vaccinia virus encoding trp-2. Depletion experiments showed that CD8+ lymphocytes and natural killer cells were crucial for the antitumor activity of the trp-2-encoding vaccines. Mice that rejected the tumor did not develop generalized vitiligo, indicating that protective immunity can be achieved in the absence of widespread autoimmune aggression.
Following an infection or immunization, a primary CD8+ T cell response generally rises then falls rapidly before giving rise to a “memory” response. When we immunized mice with recombinant viral immunogens optimized to enhance the lytic capability of CD8+ T cells, we measured a profound depression in Ag-specific effector function after early restimulation. Indeed, a “mirror image” cytolytic capability was observed: the most powerful immunogens, as measured by cytolytic capacity 6 days after immunization, elicited the weakest secondary immune response when evaluated following an additional 6 days after restimulation. To understand the mechanism of this suppression, we examined the fate of splenocytes immunized with a vaccinia virus encoding Ag and IL-2 then restimulated ex vivo. We found that these splenocytes underwent an apoptotic cell death, upon early restimulation, that was not dependent on the engagement of the FasR (CD95). Unlike previously described mechanisms of “propriocidal cell death” and “clonal exhaustion,” the cell death we observed was not an inherent property of the CD8+ T cells but rather was due to a population of splenocytes that stained positive for both the Mac-1 and Gr-1 surface markers. Deletion of these cells in vitro or in vivo completely abrogated the observed suppression of cytolytic reactivity of Ag-specific CD8+ T cells. These observations could account for the apparent absence of Ag-specific immune responses after some current vaccination regimens employing powerful immunogens. Finally, our results may shed new light on a mechanism for the suppression of CD8+ T cell responses and its effect on vaccine efficacy and on immune memory.
Tumor cells gene-modified to produce GM-CSF potently stimulate antitumor immune responses, in part, by causing the growth and differentiation of dendritic cells (DC). However, GM-CSF-modified tumor cells must be γ-irradiated or they will grow progressively, killing the host. We observed that 23 of 75 (31%) human tumor lines and two commonly used mouse tumor lines spontaneously produced GM-CSF. In mice, chronic GM-CSF production by tumors suppressed Ag-specific CD8+ T cell responses. Interestingly, an inhibitory population of adherent CD11b(Mac-1)/Gr-1 double-positive cells caused the observed impairment of CD8+ T cell function upon direct cell-to-cell contact. The inhibitory cells were positive for some markers associated with Ag presenting cells, like F4/80, but were negative for markers associated with fully mature DC like DEC205, B7.2, and MHC class II. We have previously reported that a similar or identical population of inhibitory “immature” APC was elicited after immunization with powerful recombinant immunogens. We show here that these inhibitory cells can be elicited by the administration of recombinant GM-CSF alone, and, furthermore, that they can be differentiated ex vivo into “mature” APC by the addition of IL-4 and GM-CSF. Thus, tumors may be able to escape from immune detection by producing “unopposed” GM-CSF, thereby disrupting the balance of cytokines needed for the maturation of fully functional DC. Further, CD11b/Gr-1 double-positive cells may function as “inhibitory” APC under the influence of GM-CSF alone.
Neoplastic cells are generally poor immunogens. Transfection of the murine tumor CT-26 with β-galactosidase (β-gal), a proteinfrom Escherichia coli, did not alter its growth rate in vivo, or its lethality, and did not elicit a measurable anti-β-gal immune response. Immunization with β-gal-expressing recombinant vaccinia viruses (rVV) elicited specific anti-β-gal cytolytic T lymphocytes, but rVV-β-gal was only marginally therapeutic when given to tumor-bearing mice. With the aim of expanding the immune response against β-gal, used here as a model tumor Ag, we gave mice exogenous IL-2 starting 12 h after the poxvirus. The therapeutic effectiveness of the combination of poxvirus and IL-2 was far greater than either of these treatments alone. When the cDNA for IL-2 was inserted into the viral genome of the rVV construct to make a double recombinant (drVV), antitumor activity was further augmented. One mechanism of action may be the enhanced activation or expansion of cytotoxicT cells, because a marked increase in primary cytotoxic responses against vaccinia determinants was observed. Interestingly, other cytokines (mGM-CSF, mTNF-α, and mIFN-γ) inserted into the rVV genome did not modify the efficacy of the rVV constructs. The increase in specific CTL responses against β-gal by drVV expressing the tumor-associated Ags (TAA) and IL-2 was morepronounced inmice bearing the lacZ-transduced tumor than in those bearing the parental cell line, suggesting that the TAA presented by growing tumor cells can either pre-activate or otherwise amplify the immune response induced by the rVV. Unfortunately, in several long-term surviving mice, tumor recurred that no longer expressed β-gal. These results indicate that treatment of disseminated tumors by using recombinant viruses expressing TAA can be enhanced by IL-2 provided exogenously, or encoded within the recombinant virus.