Self-specific CD8 Research Articles

Immune experiences alter tolerance in T cell-mediated autoimmunity.

Abstract Autoimmune diseases afflict >20M Americans and alarmingly the incidence of autoimmune diseases is increasing particularly in children. The hygiene hypothesis (HH) proposes that there is an inverse correlation between microbial exposure and subsequent development of autoimmunity, i.e., the “cleaner” the environment, the higher the probability that autoimmunity develops. Critically, how microbial exposures and infections alter self-antigen (Ag), a critical component of immune tolerance, is unknown. Our data show that immune-experienced mice have decreased self-specific CD8 T cell priming and organ pathology mediated by resident cells in lymph nodes (LN). Resident Ag-bearing LN stromal cells isolated from immune-experienced mice induced less self-specific CD8 T cell proliferation in vitro indicating changes in self-Ag. The HH determines that environmental stressors are important drivers of autoimmunity, yet the human immune experience is poorly modeled in SPF models of tolerance and autoimmunity. Therefore, we used mice exposed to natural mouse pathogens, "dirty" mice showed a decrease in T cell activity to self-Ag. The cohousing of NOD mice with pet shop mice resulted in zero mice developing autoimmune diabetes. Overall, our data suggest novel mechanisms by which self-antigen is altered in lymph nodes due to previous immune experiences addressing a critical mechanism behind the hygiene hypothesis, intersecting TCR reactivity with environmental impact.

MHC I tetramer staining tends to overestimate the number of functionally relevant self-reactive CD8 T cells in the preimmune repertoire.

Previous studies that used peptide-MHC (pMHC) tetramers (tet) to identify self-specific T cells have questioned the effectiveness of thymic-negative selection. Here, we used pMHCI tet to enumerate CD8 T cells specific for the immunodominant gp33 epitope of lymphocytic choriomeningitis virus glycoprotein (GP) in mice transgenically engineered to express high levels of GP as a self-antigen in the thymus. In GP-transgenic mice (GP+ ), monoclonal P14 TCR+ CD8 T cells that express a GP-specific TCR could not be detected by gp33/Db -tet staining, indicative of their complete intrathymic deletion. By contrast, in the same GP+ mice, substantial numbers of polyclonal CD8 T cells identifiable by gp33/Db -tet were present. The gp33-tet staining profiles of polyclonal T cells from GP+ and GP-negative (GP- ) mice were overlapping, but mean fluorescence intensities were ∼15% lower in cells from GP+ mice. Remarkably, the gp33-tet+ T cells in GP+ mice failed to clonally expand after lymphocytic choriomeningitis virus infection, whereas those of GP- mice did so. In Nur77GFP -reporter mice, dose-dependent responses to gp33 peptide-induced TCR stimulation revealed that gp33-tet+ T cells with highligand sensitivity are lacking in GP+ mice. Hence, pMHCI tet staining identifies self-specific CD8 T cells but tends to overestimate the number of truly self-reactive cells.

Nanoparticle-enabled innate immune stimulation activates endogenous tumor-infiltrating T cells with broad antigen specificities

Tumors are often infiltrated by T lymphocytes recognizing either self- or mutated antigens but are generally inactive, although they often show signs of prior clonal expansion. Hypothesizing that this may be due to peripheral tolerance, we formulated nanoparticles containing innate immune stimulants that we found were sufficient to activate self-specific CD8+ T cells and injected them into two different mouse tumor models, B16F10 and MC38. These nanoparticles robustly activated and/or expanded antigen-specific CD8+ tumor-infiltrating T cells, along with a decrease in regulatory CD4+ T cells and an increase in Interleukin-17 producers, resulting in significant tumor growth retardation or elimination and the establishment of immune memory in surviving mice. Furthermore, nanoparticles with modification of stimulating human T cells enabled the robust activation of endogenous T cells in patient-derived tumor organoids. These results indicate that breaking peripheral tolerance without regard to the antigen specificity creates a promising pathway for cancer immunotherapy.

CD8+ T cell self-tolerance permits responsiveness but limits tissue damage.

Self-specific CD8+T cells can escape clonal deletion, but the properties and capabilities of such cells in a physiological setting are unclear. We characterized polyclonal CD8+ T cells specific for the melanocyte antigen tyrosinase-related protein 2 (Trp2) in mice expressing or lacking this enzyme (due to deficiency in Dct, which encodes Trp2). Phenotypic and gene expression profiles of pre-immune Trp2/Kb-specific cells were similar; the size of this population was only slightly reduced in wild-type (WT) compared to Dct-deficient (Dct-/-) mice. Despite comparable initial responses to Trp2 immunization, WT Trp2/Kb-specific cells showed blunted expansion and less readily differentiated into a CD25+proliferative population. Functional self-tolerance clearly emerged when assessing immunopathology: adoptively transferred WT Trp2/Kb-specific cells mediated vitiligo much less efficiently. Hence, CD8+ T cell self-specificity is poorly predicted by precursor frequency, phenotype, or even initial responsiveness, while deficient activation-induced CD25 expression and other gene expression characteristics may help to identify functionally tolerant cells.

Melanocyte-specific CD8+ T cells demonstrate tolerance characterized by an impaired ability to differentiate into a highly proliferative population

Abstract The mechanisms governing non-deletional CD8+ T cell tolerance are poorly understood. We are using a physiologically-relevant mouse model to better understand the regulation of self-specific CD8+ T cells, which impact both autoimmune disease and cancer. We find a substantial number of polyclonal cells specific for an epitope from the melanocyte/melanoma-associated enzyme tyrosinase-related protein 2 (Trp2) in both wild-type (WT) C57BL/6 and Trp2-deficient (Trp2 KO) mice. In pre-immune mice, these cells have a naïve phenotype in both strains, and they respond similarly for the first several days after in vivo stimulation with Trp2 peptide alone or in combination with adjuvants (TriVax). However, significantly fewer WT Trp2-specific cells are present at an effector timepoint after TriVax or recombinant Trp2-expressing Listeria monocytogenes relative to KO Trp2-specific cells. WT Trp2-specific cells also show a deficiency in mediating vitiligo compared with KO cells when TriVax-generated effectors are transferred into recipient WT mice primed with TriVax and dinitrofluorobenzene (DNFB). Single-cell RNA sequencing at day three after stimulation reveals that KO Trp2-specific cells efficiently differentiate into a proliferative population, whereas few WT Trp2-specific cells demonstrate this phenotype. As a population, WT Trp2-specific CD8+ T cells exhibit covert anergy: despite a naïve appearance at steady state and initial acquisition of activation markers after stimulation, these self-specific cells are unable to expand productively or to efficiently mediate functional anti-melanocyte activity. These findings have implications for recently-described non-deletional CD8+ T cell tolerance in humans.

Eomes identifies thymic precursors of self-specific memory-phenotype CD8+ T cells.

Unprimed mice harbor a substantial population of "memory-phenotype" CD8+ T cells (CD8-MP cells) that exhibit hallmarks of activation and innate-like functional properties. Due to the lack of faithful markers to distinguish CD8-MP cells from bona fide CD8+ memory T cells, the developmental origins and antigen specificities of CD8-MP cells remain incompletely defined. Using deep T cell antigen receptor (TCR) sequencing, we found that the TCRs expressed by CD8-MP cells are highly recurrent and distinct from the TCRs expressed by naive-phenotype CD8+ T cells. CD8-MP clones exhibited reactivity to widely expressed self-ligands. T cell precursors expressing CD8-MP TCRs upregulated the transcription factor Eomes during maturation in the thymus, prior to induction of the full memory phenotype, suggestive of a unique program triggered by recognition of self-ligands. Moreover, CD8-MP cells infiltrate oncogene-driven prostate tumors and express high densities of PD-1, suggesting a potential role in anti-tumor immunity and response to immunotherapy.

Robust Iterative Stimulation with Self-Antigens Overcomes CD8+ T Cell Tolerance to Self- and Tumor Antigens

SUMMARYThe immune system adapts to constitutive antigens to preserve self-tolerance, which is a major barrier for anti-tumor immunity. Antigen-specific reversal of tolerance constitutes a major goal to spur therapeutic applications. Here, we show that robust, iterative, systemic stimulation targeting tissue-specific antigens in the context of acute infections reverses established CD8+ T cell tolerance to self, including in T cells that survive negative selection. This strategy results in large numbers of circulating and resident memory self-specific CD8+ T cells that are widely distributed and can be co-opted to control established malignancies bearing self-antigen without concomitant autoimmunity. Targeted expansion of both self- and tumor neoantigen-specific T cells acts synergistically to boost anti-tumor immunity and elicits protection against aggressive melanoma. Our findings demonstrate that T cell tolerance can be re-adapted to responsiveness through robust antigenic exposure, generating self-specific CD8+ T cells that can be used for cancer treatment.

Negligible Role for Deletion Mediated by cDC1 in CD8+ T Cell Tolerance.

Deletion of CD8+ T cells by dendritic cells (DCs) is recognized as a critical mechanism of immune tolerance to self-antigens. Although DC-mediated peripheral deletion of autoreactive CD8+ T cells has been demonstrated using T cells reactive to model Ags, its role in shaping the naturally occurring polyclonal CD8+ T cell repertoire has not been defined. Using Batf3-/- mice lacking cross-presenting CD8α+ and CD103+ DCs (also known as type 1 conventional [cDC1]), we demonstrate that peripheral deletion of CD8+ T cells reactive to a model tissue Ag is dependent on cDC1. However, endogenous CD8+ T cells from the periphery of Batf3-/- mice do not exhibit heightened self-reactivity, and deep TCR sequencing of CD8+ T cells from Batf3-/- and Batf3+/+ mice reveals that cDC1 have a minimal impact on shaping the peripheral CD8+ T cell repertoire. Thus, although evident in reductionist systems, deletion of polyclonal self-specific CD8+ T cells by cDC1 plays a negligible role in enforcing tolerance to natural self-ligands.

Abstract B142: Activation of endogenous anergic self-specific CD8+ T-cells by polymeric nanoparticles for enhanced cancer immunotherapy

Abstract While the human immune system is often able to protect the body from infectious pathogens, it has multiple mechanisms to inhibit mounting an immune system against what it perceives as “self” and thus often fails to eliminate cancer cells since they seem to have the characteristics of “self.” Our lab found that cytotoxic CD8+ T-cells that recognize self antigens in the peripheral blood of healthy humans were present in frequencies similar to those specific for non-self antigens, but these self-specific CD8+ T-cells are resistant to activation and/or expansion. We hypothesized that tumor-infiltrating CD8+ T-cells recognizing self antigens are often in an anergized state but they could be activated with the appropriate immunostimulatory signals to augment antitumor immunity. Thus, we developed a poly(lactide-co-glycolide) nanoparticle (PLGA NP) based immunostimulatory platform that is surface-functionalized with monoclonal anti-CD28 antibody (Ab), and encapsulated interleukin-2 (IL-2), toll-like receptor-2 (TLR2) agonist and nucleotide-binding oligomerization domain-containing protein 2 (NOD2) in the hydrophobic core. This platform enables the encapsulated immunostimulatory drugs to be continuously released in the tumor microenvironment to activate the tumor infiltrating self-specific CD8+ T-cells in order to enhance antitumor immune response while minimizing the rapid diffusion of these drugs into other parts of the body, greatly reducing the systemic toxicities. Our results demonstrated that the mice bearing melanoma or colon cancers treated with the stimulatory PLGA NP have elicited potent T-cell response, resulting in marked tumor regression and increased overall survival. The developed PLGA NPs are also capable of generating an immunologic memory that prevented tumor growth after rechallenging the survivor mice with the same cancer cell type. Moreover, the developed PLGA NPs can be combined with checkpoint blocking antibodies to achieve the synergistic effect, significantly prolonging the overall survival. We also applied the single-cell TCR sequencing technique to assess both T-cell receptor and phenotypic characteristics of tumor-infiltrating CD8+ T-cells after NP stimulation. The result was further combined with yeast-display libraries of peptide-MHC to identify tumor-specific antigens. Citation Format: Qian Yin. Activation of endogenous anergic self-specific CD8+ T-cells by polymeric nanoparticles for enhanced cancer immunotherapy [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr B142.

Reprogramming responsiveness to checkpoint blockade in dysfunctional CD8 T cells

Established T cell dysfunction is a barrier to antitumor responses, and checkpoint blockade presumably reverses this. Many patients fail to respond to treatment and/or develop autoimmune adverse events. The underlying reason for T cell responsiveness remains elusive. Here, we show that susceptibility to checkpoint blockade is dependent on the activation status of T cells. Newly activated self-specific CD8 T cells respond to checkpoint blockade and cause autoimmunity, which is mitigated by inhibiting the mechanistic target of rapamycin. However, once tolerance is established, self-specific CD8 T cells display a gene signature comparable to tumor-specific CD8 T cells in a fixed state of dysfunction. Tolerant self-specific CD8 T cells do not respond to single or combinatorial dosing of anti-CTLA4, anti-PD-L1, anti-PD-1, anti-LAG-3, and/or anti-TIM-3. Despite this, T cell responsiveness can be induced by vaccination with cognate antigen, which alters the previously fixed transcriptional signature and increases antigen-sensing machinery. Antigenic reeducation of tolerant T cells synergizes with checkpoint blockade to generate functional CD8 T cells, which eliminate tumors without concomitant autoimmunity and are transcriptionally distinct from classic effector T cells. These data demonstrate that responses to checkpoint blockade are dependent on the activation state of a T cell and show that checkpoint blockade-insensitive CD8 T cells can be induced to respond to checkpoint blockade with robust antigenic stimulation to participate in tumor control.

Utilizing self-specific CD8 T cells for the treatment of cancer

Abstract CD8 T cells are potent effectors that monitor the body for infections as well as malignancies. CD8 T cell infiltration into tumors is correlated with better prognoses in many cancers. However, the tumor microenvironment is often toleragenic, which treatments like checkpoint inhibitors seek to overcome. Such treatments have been met with some success, but their benefits remain elusive for a subset of patients. Using a strict model of tolerance to intestinal self-antigen, we show that newly activated CD8 T cells respond best to checkpoint blockade therapy, while CD8 T cells in a toleragenic environment, even for a short time, do not respond. We have developed a therapeutic vaccination strategy that generates a targeted CD8 T cell response with renewed susceptibility to checkpoint blockade. Using heterologous prime boost vaccination model, we have refined tolerance reversal to a subset of self-antigen specific CD8 T cells, and eliminated the need for broadly immunostimulatory treatments. This model has been used to reverse tolerance in CD8 T cells specific for engineered antigens in the small intestine as well as naturally occurring self-antigens in the skin. The expanded populations of skin-specific CD8 T cells can be utilized for the treatment of melanoma. These melanocyte-specific CD8 T cells are functional and generate tissue resident memory (Trm) populations throughout the body, including the skin. Additionally, melanocyte-specific CD8 T cells can cooperate with neo-antigen specific CD8 T cells to aid in tumor clearance. By investigating tolerance reversal in self-specific CD8 T cells we have uncovered ways to improve upon current anti-cancer treatments and developed potentially translatable anti-cancer vaccine strategies.

Visualizing antigen-specific CD8 T cell dynamics in the small intestine

Abstract The small intestine (SI) represents the largest mucosal barrier between the environment and the host. As such, the SI limits both normal and pathogenic microbe invasion and must balance immune tolerance and activation. CD8ab T cells constitute a dominant lymphocyte population in the SI and contribute to both of these processes but detailed investigation into their in situ behavior in response to various stimuli is lacking. We visualized Ag-specific CD8 T cell migration in the SI using two-photon microscopy to understand CD8 T cell immunosurveillance. We found that pathogen-specific CD8 T cells are initially slowed early after infection but regain motility directly after Ag clearance. At a memory time point, CD8 T cells exhibited slower migration concurrent with acquisition of a resident memory phenotype. Current studies are focused on determining what is required for CD8 T cell motility throughout the course of infection. Additionally, as tolerance reversal and expansion of self-specific CD8 T cells has recently become a focus of cancer therapy, we characterized in situ behavior of self-specific CD8 T cells. Using a mouse expressing a model Ag in the SI as a self-protein, we observed self-specific CD8 T cells before and after tolerance reversal. Tolerant CD8 T cells migrate significantly faster than pathogen-specific cells at acute time points after infection. Contrastingly, ex-tolerant CD8 T cells were comparable to pathogen-specific CD8 T cells indicating that indeed their in situ scanning behavior had been altered upon tolerance reversal. These observations have provided valuable insight into CD8 T cell motility within the SI and can inform targeted treatment options and vaccines to ensure effective immunosurveillance of this tissue.

Clonal Deletion Prunes but Does Not Eliminate Self-Specific αβ CD8+ T Lymphocytes

It has long been thought that clonal deletion efficiently removes almost all self-specific T cells from the peripheral repertoire. We found that self-peptide MHC-specific CD8(+) T cells in the blood of healthy humans were present in frequencies similar to those specific for non-self antigens. For the Y chromosome-encoded SMCY antigen, self-specific T cells exhibited only a 3-fold lower average frequency in males versus females and were anergic with respect to peptide activation, although this inhibition could be overcome by a stronger stimulus. We conclude that clonal deletion prunes but does not eliminate self-specific T cells and suggest that to do so would create holes in the repertoire that pathogens could readily exploit. In support of this hypothesis, we detected T cells specific for all 20 amino acid variants at the p5 position of a hepatitis C virus epitope in a random group of blood donors.

Repeated antigen exposure reverses endogenous CD8 T cell tolerance and is potentiated by IL-4 (MUC8P.804)

Abstract Elucidating the induction, maintenance, and reversal of self-tolerant CD8 T cells is imperative for understanding the development and progression of autoimmunity, as well as cancer. Using a mouse model that permits examination of responses to small intestinal self-antigen, we can evaluate endogenous polyclonal self-specific T cells that have undergone tolerance induction. This established CD8 T cell tolerance can be reversed after repeated antigen delivery, generating large numbers of activated, functional endogenous self-specific T cells. Interestingly, this response is reduced in IL-4 limiting conditions suggesting a mechanistic role for IL-4 in self-reactive CD8 T cells responses. We hypothesize that IL-4 is required for self-specific CD8 T cell selection, survival and/or responses to cognate antigen. We show that while IL-4 does not act directly on CD8 T cells, it does impact their proliferation, especially during Ag recognition. Interestingly, we can detect self-reactive T cells that have escaped thymic selection in unmanipulated mice, but these remain naïve, despite the presence of large amounts of cognate MHC:Ag complexes. Future studies will focus on elucidating the function and origin of IL-4 in self-specific responses and understanding properties of endogenous, polyclonal self-specific CD8 T cells in negatively selecting environments.

PD-1 and CTLA-4 signals are not important for the maintenance of tolerant self-reactive CD8 T cells in the small intestine (MUC8P.808)

Abstract Tolerance is an important mechanism by which the immune system distinguishes between self and non-self. Peripheral T cell tolerance is critical for the control of self-reactive cells that escape negative selection. Anergy is one mechanism that controls self-reactive cells in the periphery and prevents autoimmunity. Using a system that allows for the study self-specific CD8 T cell responses to a known intestinal auto-antigen, we determined that mucosal antigen signaling can lead to a long-term, stable population of cells in the intestine that have a classic anergic phenotype. These cells do not produce effector cytokines or induce intestinal damage. Inhibitory molecules, like PD-1 and CTLA-4, are believed to be important for both the induction and maintenance of T cell tolerance. We show that the induction phase of CD8 T cell tolerance to self was highly sensitive to control through both PD-1 and CTLA-4. However, once tolerance was established, the maintenance of tolerant CD8 T cells was independent of PD-1 or CTLA-4 signaling. Overall, this indicates that distinct mechanisms are responsible for the induction and maintenance of tolerant T cells. In addition, there may be many tolerant T cells resistant to immunotherapeutics targeting these pathways. Determining which subsets of self-reactive cells are susceptible to modulation through these pathways is important for understanding basic mechanisms tolerance and autoimmunity and for improving cancer therapeutics.

The ThPOK transcription factor differentially affects the development and function of self-specific CD8+T cells and regulatory CD4+T cells

The zinc finger transcription factor ThPOK plays a crucial role in CD4 T-cell development and CD4/CD8 lineage decision. In ThPOK-deficient mice, developing T cells expressing MHC class II-restricted T-cell receptors are redirected into the CD8 T-cell lineage. In this study, we investigated whether the ThPOK transgene affected the development and function of two additional types of T cells, namely self-specific CD8 T cells and CD4(+) FoxP3(+) T regulatory cells. Self-specific CD8 T cells are characterized by high expression of CD44, CD122, Ly6C, 1B11 and proliferation in response to either IL-2 or IL-15. The ThPOK transgene converted these self-specific CD8 T cells into CD4 T cells. The converted CD4(+) T cells are no longer self-reactive, lose the characteristics of self-specific CD8 T cells, acquire the properties of conventional CD4 T cells and survive poorly in peripheral lymphoid organs. By contrast, the ThPOK transgene promoted the development of CD4(+) FoxP3(+) regulatory T cells resulting in an increased recovery of CD4(+) FoxP3(+) regulatory T cells that expressed higher transforming growth factor-β-dependent suppressor activity. These studies indicate that the ThPOK transcription factor differentially affects the development and function of self-specific CD8 T cells and CD4(+) FoxP3(+) regulatory T cells.

THU0046 CD8+ T Cell Reactivity Against Apoptotic Self-Epitopes in Rheumatoid Arthritis Patients Treated with Etanercept

BackgroundApoptosis seems to play a key role in autoimmune diseases (1). We have previously shown that T cell apoptosis generates a wide array of caspase-cleaved proteins, which can be cross-presented...

Positive selection of self‐antigen‐specific CD8+ T cells by hematopoietic cells

In contrast to thymic epithelial cells, which induce the positive selection of conventional CD8(+) T cells, hematopoietic cells (HCs) select innate CD8(+) T cells whose Ag specificity is not fully understood. Here we show that CD8(+) T cells expressing an H-Y Ag-specific Tg TCR were able to develop in mice in which only HCs expressed MHC class I, when HCs also expressed the H-Y Ag. These HC-selected self-specific CD8(+) T cells resemble innate CD8(+) T cells in WT mice in terms of the expression of memory markers and effector functions, but are phenotypically distinct from the thymus-independent CD8(+) T-cell population. The peripheral maintenance of H-Y-specific CD8(+) T cells required presentation of the self-Ag and IL-15 on HCs. HC-selected CD8(+) T cells in mice lacking the Tg TCR also showed these features. Furthermore, by using MHC class I tetramers with a male Ag peptide, we found that self-Ag-specific CD8(+) T cells in TCR non-Tg mice could develop via HC-induced positive selection, supporting results obtained from H-Y TCR Tg mice. These findings indicate the presence of self-specific CD8(+) T cells that are positively selected by HCs in the peripheral T-cell repertoire.

The cKrox transgene affects the development and function of multiple T cell types. (64.10)

Abstract The zinc finger transcription factor cKrox is crucial for CD4 T cell development. Mice lacking a functional cKrox gene lack CD4 T cells. Furthermore, in cKrox-deficient mice, developing T cells expressing a MHC class II-restricted TCR are redirected into the CD8 T cell lineage. We investigated the effect of the cKrox transgene on the development and function of two other types of T cells, namely self-specific CD8 T cells and CD4 T regulatory cells. In TCR transgenic mice that express a male-specific TCR (H-Y TCR), the only CD8 T cells that developed in male TCR transgenic mice are self-specific CD8 T cells. These H-Y TCR+ self-specific CD8 T cells are characterized by high expression of CD44, CD122, Ly6C, 1B11, NK cell markers and proliferative response in response to IL-2 or IL-15. We found that the cKrox transgene converted these self-specific CD8 T cells into CD4 T cells. Furthermore, the converted CD4 T cells lost the characteristics of these self-specific CD8 T cells described above. Notably, the converted self-specific CD8 T cells express genes that are characteristic of CD4 T cells (GATA-3 and IL-2). The numbers of CD4 T regulatory cells were significantly higher in cKrox transgenic mice and these cells expressed higher suppressor activity compared to CD4 T regulatory cells from control mice. These studies indicate that the cKrox transgene affects the development and function of multiple T cell types. This work is supported by the Canadian Cancer Society.

Melanoma, Darwinian medicine and the inner world.

IntroductionA diverse range of human diseases, including allergy, asthma, autoimmune disease, cancer and chronic neurologic diseases, notably multiple sclerosis and endogenous depression, is becoming more prevalent in industrialized countries. It has been postulated that environmental factors associated with improved standards of hygiene play a leading role in this process since the immune system seems to need extrinsic challenges for its proper maturation.The inner worldAn added dimension has now emerged—the impact on disease of the inner world, principally the numerous endogenous retroviruses (HERVs) within the human genome. Taking melanoma as an example, we propose a framework for understanding how a complex infectious and immunological background can induce or inhibit expression of a HERV-related disease process. The central role of a failure to induce or to maintain certain populations of self-specific CD8+ T-cells mediating immune surveillance, the expression of HERV-encoded peptides on affected cells and pathological mechanisms directly attributable to HERV proteins are discussed.ConclusionsThe presented concepts explain events preceding the clinical manifestation of diseases by several years and provide a rationale for the use of currently available vaccines to protect against certain HERV-induced diseases, especially melanoma. Criteria for establishing the causal role of HERVs in a given disease are proposed.