CD1 lipidomes reveal lipid-binding motifs and size-based antigen-display mechanisms.

The CD1 system binds lipid antigens for display to T cells. Here, we solved lipidomes for the four human CD1 antigen-presenting molecules, providing a map of self-lipid display. Answering a basic question, the detection of >2,000 CD1-lipid complexes demonstrates broad presentation of self-sphingolipids and phospholipids. Whereas peptide antigens are chemically processed, many lipids are presented in an unaltered form. However, each type of CD1 protein differentially edits the self-lipidome to show distinct capture motifs based on lipid length and chemical composition, suggesting general antigen display mechanisms. For CD1a and CD1d, lipid size matches the CD1 cleft volume. CD1c cleft size is more variable, and CD1b is the outlier, where ligands and clefts show an extreme size mismatch that is explained by uniformly seating two small lipids in one cleft. Furthermore, the list of compounds that comprise the integrated CD1 lipidome supports the ongoing discovery of lipid blockers and antigens for T cells.

Spheromers reveal robust T cell responses to the Pfizer/BioNTech vaccine and attenuated peripheral CD8+ T cell responses post SARS-CoV-2 infection.

T cells are a critical component of the response to SARS-CoV-2, but their kinetics after infection and vaccination are insufficiently understood. Using "spheromer" peptide-MHC multimer reagents, we analyzed healthy subjects receiving two doses of the Pfizer/BioNTech BNT162b2 vaccine. Vaccination resulted in robust spike-specific T cell responses for the dominant CD4+ (HLA-DRB1∗15:01/S191) and CD8+ (HLA-A∗02/S691) T cell epitopes. Antigen-specific CD4+ and CD8+ T cell responses were asynchronous, with the peak CD4+ T cell responses occurring 1 week post the second vaccination (boost), whereas CD8+ T cells peaked 2 weeks later. These peripheral T cell responses were elevated compared with COVID-19 patients. We also found that previous SARS-CoV-2 infection resulted in decreased CD8+ T cell activation and expansion, suggesting that previous infection can influence the T cell response to vaccination.

T cell autoreactivity directed toward CD1c itself rather than toward carried self lipids.

The hallmark function of αß T cell antigen receptors (TCRs) involves the highly specific co-recognition of a major histocompatibility complex molecule and its carried peptide. However, the molecular basis of the interactions of TCRs with the lipid antigen-presenting molecule CD1c is unknown. We identified frequent staining of human T cells with CD1c tetramers across numerous subjects. Whereas TCRs typically show high specificity for antigen, both tetramer binding and autoreactivity occurred with CD1c in complex with numerous, chemically diverse self lipids. Such extreme polyspecificity was attributable to binding of the TCR over the closed surface of CD1c, with the TCR covering the portal where lipids normally protrude. The TCR essentially failed to contact lipids because they were fully seated within CD1c. These data demonstrate the sequestration of lipids within CD1c as a mechanism of autoreactivity and point to small lipid size as a determinant of autoreactive T cell responses.

CD1a-autoreactive T cells recognize natural skin oils that function as headless antigens.

T cells autoreactive to the antigen-presenting molecule CD1a are common in human blood and skin, but the search for natural autoantigens has been confounded by background T cell responses to CD1 proteins and self lipids. After capturing CD1a-lipid complexes, we gently eluted ligands while preserving non-ligand-bound CD1a for testing lipids from tissues. CD1a released hundreds of ligands of two types. Inhibitory ligands were ubiquitous membrane lipids with polar head groups, whereas stimulatory compounds were apolar oils. We identified squalene and wax esters, which naturally accumulate in epidermis and sebum, as autoantigens presented by CD1a. The activation of T cells by skin oils suggested that headless mini-antigens nest within CD1a and displace non-antigenic resident lipids with large head groups. Oily autoantigens naturally coat the surface of the skin; thus, this points to a previously unknown mechanism of barrier immunity.

A conserved human T cell population targets mycobacterial antigens presented by CD1b.

Human T cell antigen receptors (TCRs) pair in millions of combinations to create complex and unique T cell repertoires for each person. Through the use of tetramers to analyze TCRs reactive to the antigen-presenting molecule CD1b, we detected T cells with highly stereotyped TCR α-chains present among genetically unrelated patients with tuberculosis. The germline-encoded, mycolyl lipid-reactive (GEM) TCRs had an α-chain bearing the variable (V) region TRAV1-2 rearranged to the joining (J) region TRAJ9 with few nontemplated (N)-region additions. Analysis of TCRs by high-throughput sequencing, binding and crystallography showed linkage of TCRα sequence motifs to high-affinity recognition of antigen. Thus, the CD1-reactive TCR repertoire is composed of at least two compartments: high-affinity GEM TCRs, and more-diverse TCRs with low affinity for CD1b-lipid complexes. We found high interdonor conservation of TCRs that probably resulted from selection by a nonpolymorphic antigen-presenting molecule and an immunodominant antigen.

Minimal Information about MHC Multimers (MIAMM).

With the goal of improving the reproducibility and annotatability of MHC multimer reagent data, we present the establishment of a new data standard: Minimal Information about MHC Multimers (https://miamm.lji.org/). Multimers are engineered reagents composed of a ligand and a MHC, which can be represented in a standardized format using ontology terminology. We provide an online Web site to host the details of the standard, as well as a validation tool to assist with the adoption of the standard. We hope that this publication will bring increased awareness of Minimal Information about MHC Multimers and drive acceptance, ultimately improving the quality and documentation of multimer data in the scientific literature.

T Cells Specific for a Mycobacterial Glycolipid Expand after Intravenous Bacillus Calmette-Guérin Vaccination.

Intradermal vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) protects infants from disseminated tuberculosis, and i.v. BCG protects nonhuman primates (NHP) against pulmonary and extrapulmonary tuberculosis. In humans and NHP, protection is thought to be mediated by T cells, which typically recognize bacterial peptide Ags bound to MHC proteins. However, during vertebrate evolution, T cells acquired the capacity to recognize lipid Ags bound to CD1a, CD1b, and CD1c proteins expressed on APCs. It is unknown whether BCG induces T cell immunity to mycobacterial lipids and whether CD1-restricted T cells are resident in the lung. In this study, we developed and validated Macaca mulatta (Mamu) CD1b and CD1c tetramers to probe ex vivo phenotypes and functions of T cells specific for glucose monomycolate (GMM), an immunodominant mycobacterial lipid Ag. We discovered that CD1b and CD1c present GMM to T cells in both humans and NHP. We show that GMM-specific T cells are expanded in rhesus macaque blood 4 wk after i.v. BCG, which has been shown to protect NHP with near-sterilizing efficacy upon M. tuberculosis challenge. After vaccination, these T cells are detected at high frequency within bronchoalveolar fluid and express CD69 and CD103, markers associated with resident memory T cells. Thus, our data expand the repertoire of T cells known to be induced by whole cell mycobacterial vaccines, such as BCG, and show that lipid Ag-specific T cells are resident in the lungs, where they may contribute to protective immunity.

Casting a wider net: Immunosurveillance by nonclassical MHC molecules.

Most studies of T lymphocytes focus on recognition of classical major histocompatibility complex (MHC) class I or II molecules presenting oligopeptides, yet there are numerous variations and exceptions of biological significance based on recognition of a wide variety of nonclassical MHC molecules. These include αß and γδ T cells that recognize different class Ib molecules (CD1, MR-1, HLA-E, G, F, et al.) that are nearly monomorphic within a given species. Collectively, these T cells can be considered "unconventional," in part because they recognize lipids, metabolites, and modified peptides. Unlike classical MHC-specific cells, unconventional T cells generally exhibit limited T-cell antigen receptor (TCR) repertoires and often produce innate immune cell-like rapid effector responses. Exploiting this system in new generation vaccines for human immunodeficiency virus (HIV), tuberculosis (TB), other infectious agents, and cancer was the focus of a recent workshop, "Immune Surveillance by Non-classical MHC Molecules: Improving Diversity for Antigens," sponsored by the National Institute of Allergy and Infectious Diseases. Here, we summarize salient points presented regarding the basic immunobiology of unconventional T cells, recent advances in methodologies to measure unconventional T-cell activity in diseases, and approaches to harness their considerable clinical potential.

The Frequency of Vaccine-Induced T-Cell Responses Does Not Predict the Rate of Acquisition after Repeated Intrarectal SIVmac239 Challenges in Mamu-B*08+ Rhesus Macaques.

Approximately 50% of rhesus macaques (RMs) expressing the major histocompatibility complex class I (MHC-I) allele Mamu-B*08 spontaneously control chronic-phase viremia after infection with the pathogenic simian immunodeficiency virus mac239 (SIVmac239) clone. CD8+ T-cell responses in these animals are focused on immunodominant Mamu-B*08-restricted SIV epitopes in Vif and Nef, and prophylactic vaccination with these epitopes increases the incidence of elite control in SIVmac239-infected Mamu-B*08-positive (Mamu-B*08+ ) RMs. Here we evaluated if robust vaccine-elicited CD8+ T-cell responses against Vif and Nef can prevent systemic infection in Mamu-B*08+ RMs following mucosal SIV challenges. Ten Mamu-B*08+ RMs were vaccinated with a heterologous prime/boost/boost regimen encoding Vif and Nef, while six sham-vaccinated MHC-I-matched RMs served as the controls for this experiment. Vaccine-induced CD8+ T cells against Mamu-B*08-restricted SIV epitopes reached high frequencies in blood but were present at lower levels in lymph node and gut biopsy specimens. Following repeated intrarectal challenges with SIVmac239, all control RMs became infected by the sixth SIV exposure. By comparison, four vaccinees were still uninfected after six challenges, and three of them remained aviremic after 3 or 4 additional challenges. The rate of SIV acquisition in the vaccinees was numerically lower (albeit not statistically significantly) than that in the controls. However, peak viremia was significantly reduced in infected vaccinees compared to control animals. We found no T-cell markers that distinguished vaccinees that acquired SIV infection from those that did not. Additional studies will be needed to validate these findings and determine if cellular immunity can be harnessed to prevent the establishment of productive immunodeficiency virus infection.IMPORTANCE It is generally accepted that the antiviral effects of vaccine-induced classical CD8+ T-cell responses against human immunodeficiency virus (HIV) are limited to partial reductions in viremia after the establishment of productive infection. Here we show that rhesus macaques (RMs) vaccinated with Vif and Nef acquired simian immunodeficiency virus (SIV) infection at a lower (albeit not statistically significant) rate than control RMs following repeated intrarectal challenges with a pathogenic SIV clone. All animals in the present experiment expressed the elite control-associated major histocompatibility complex class I (MHC-I) molecule Mamu-B*08 that binds immunodominant epitopes in Vif and Nef. Though preliminary, these results provide tantalizing evidence that the protective efficacy of vaccine-elicited CD8+ T cells may be greater than previously thought. Future studies should examine if vaccine-induced cellular immunity can prevent systemic viral replication in RMs that do not express MHC-I alleles associated with elite control of SIV infection.

A High Throughput Whole Blood Assay for Analysis of Multiple Antigen-Specific T Cell Responses in Human Mycobacterium tuberculosis Infection.

Antigen-specific CD4 and CD8 T cells are important components of the immune response to Mycobacterium tuberculosis, yet little information is currently known regarding how the breadth, specificity, phenotype, and function of M. tuberculosis-specific T cells correlate with M. tuberculosis infection outcome in humans. To facilitate evaluation of human M. tuberculosis-specific T cell responses targeting multiple different Ags, we sought to develop a high throughput and reproducible T cell response spectrum assay requiring low blood sample volumes. We describe here the optimization and standardization of a microtiter plate-based, diluted whole blood stimulation assay utilizing overlapping peptide pools corresponding to a functionally diverse panel of 60 M. tuberculosis Ags. Using IFN-γ production as a readout of Ag specificity, the assay can be conducted using 50 µl of blood per test condition and can be expanded to accommodate additional Ags. We evaluated the intra- and interassay variability, and implemented testing of the assay in diverse cohorts of M. tuberculosis-unexposed healthy adults, foreign-born adults with latent M. tuberculosis infection residing in the United States, and tuberculosis household contacts with latent M. tuberculosis infection in a tuberculosis-endemic setting in Kenya. The M. tuberculosis-specific T cell response spectrum assay further enhances the immunological toolkit available for evaluating M. tuberculosis-specific T cell responses across different states of M. tuberculosis infection, and can be readily implemented in resource-limited settings. Moreover, application of the assay to longitudinal cohorts will facilitate evaluation of treatment- or vaccine-induced changes in the breadth and specificity of Ag-specific T cell responses, as well as identification of M. tuberculosis-specific T cell responses associated with M. tuberculosis infection outcomes.

Mamu-B*17+ Rhesus Macaques Vaccinated with env, vif, and nef Manifest Early Control of SIVmac239 Replication.

Certain major histocompatibility complex class I (MHC-I) alleles are associated with spontaneous control of viral replication in human immunodeficiency virus (HIV)-infected people and simian immunodeficiency virus (SIV)-infected rhesus macaques (RMs). These cases of "elite" control of HIV/SIV replication are often immune-mediated, thereby providing a framework for studying anti-lentiviral immunity. In this study, we examined how vaccination impacts SIV replication in RMs expressing the MHC-I allele Mamu-B*17 Approximately 21% of Mamu-B*17+ and 50% of Mamu-B*08+ RMs control chronic-phase viremia after SIVmac239 infection. Because CD8+ T cells targeting Mamu-B*08-restricted SIV epitopes have been implicated in virologic suppression in Mamu-B*08+ RMs, we investigated whether this might also be true for Mamu-B*17+ RMs. Two groups of Mamu-B*17+ RMs were vaccinated with genes encoding Mamu-B*17-restricted epitopes in Vif and Nef. These genes were delivered by themselves (group 1) or together with env (group 2). Group 3 included MHC-I-matched RMs and served as the control group. Surprisingly, the group 1 vaccine regimen had little effect on viral replication compared to group 3, suggesting that unlike Mamu-B*08+ RMs, preexisting SIV-specific CD8+ T cells alone do not facilitate long-term virologic suppression in Mamu-B*17+ RMs. Remarkably, however, 5/8 group 2 vaccinees controlled viremia to <15 viral RNA copies/ml soon after infection. No serological neutralizing activity against SIVmac239 was detected in group 2, although vaccine-elicited gp140-binding antibodies correlated inversely with nadir viral loads. Collectively, these data shed new light on the unique mechanism of elite control in Mamu-B*17+ RMs and implicate vaccine-induced, nonneutralizing anti-Env antibodies in the containment of immunodeficiency virus infection.IMPORTANCE A better understanding of the immune correlates of protection against HIV might facilitate the development of a prophylactic vaccine. Therefore, we investigated simian immunodeficiency virus (SIV) infection outcomes in rhesus macaques expressing the major histocompatibility complex class I allele Mamu-B*17 Approximately 21% of Mamu-B*17+ macaques spontaneously controlled chronic phase viremia after SIV infection, an effect that may involve CD8+ T cells targeting Mamu-B*17-restricted SIV epitopes. We vaccinated Mamu-B*17+ macaques with genes encoding immunodominant epitopes in Vif and Nef alone (group 1) or together with env (group 2). Although neither vaccine regimen prevented SIV infection, 5/8 group 2 vaccinees controlled viremia to below detection limits shortly after infection. This outcome, which was not observed in group 1, was associated with vaccine-induced, nonneutralizing Env-binding antibodies. Together, these findings suggest a limited contribution of Vif- and Nef-specific CD8+ T cells for virologic control in Mamu-B*17+ macaques and implicate anti-Env antibodies in containment of SIV infection.

Vaccine-induced immune responses against both Gag and Env improve control of simian immunodeficiency virus replication in rectally challenged rhesus macaques.

The ability to control lentivirus replication may be determined, in part, by the extent to which individual viral proteins are targeted by the immune system. Consequently, defining the antigens that elicit the most protective immune responses may facilitate the design of effective HIV-1 vaccines. Here we vaccinated four groups of rhesus macaques with a heterologous vector prime/boost/boost/boost (PBBB) regimen expressing the following simian immunodeficiency virus (SIV) genes: env, gag, vif, rev, tat, and nef (Group 1); env, vif, rev, tat, and nef (Group 2); gag, vif, rev, tat, and nef (Group 3); or vif, rev, tat, and nef (Group 4). Following repeated intrarectal challenges with a marginal dose of the neutralization-resistant SIVmac239 clone, vaccinees in Groups 1-3 became infected at similar rates compared to control animals. Unexpectedly, vaccinees in Group 4 became infected at a slower pace than the other animals, although this difference was not statistically significant. Group 1 exhibited the best post-acquisition virologic control of SIV infection, with significant reductions in both peak and chronic phase viremia. Indeed, 5/8 Group 1 vaccinees had viral loads of less than 2,000 vRNA copies/mL of plasma in the chronic phase. Vaccine regimens that did not contain gag (Group 2), env (Group 3), or both of these inserts (Group 4) were largely ineffective at decreasing viremia. Thus, vaccine-induced immune responses against both Gag and Env appeared to maximize control of immunodeficiency virus replication. Collectively, these findings are relevant for HIV-1 vaccine design as they provide additional insights into which of the lentiviral proteins might serve as the best vaccine immunogens.

MHC-I peptide binding activity assessed by exchange after cleavage of peptide covalently linked to ß2-microglobulin.

A common approach to measuring binding constants involves combining receptor and ligand and measuring the distribution of bound and free states after equilibration. For class I major histocompatibility (MHC-I) proteins, which bind short peptides for presentation to T cells, this approach is precluded by instability of peptide-free protein. Here we develop a method wherein a weakly-binding peptide covalently attached to the N-terminus of the MHC-I ß2m subunit is released from the peptide binding site after proteolytic cleavage of the linker. The resultant protein is able to bind added peptide. A direct binding assay and method for estimation of peptide binding constant (Kd) are described, in which fluorescence polarization is used to follow peptide binding. A competition binding assay and method for estimation of inhibitor binding constant (Ki) using the same principle also are also described. The method uses a cubic equation to relate observed binding to probe concentration, probe Kd, inhibitor concentration, and inhibitor Ki under general reaction conditions without assumptions relating to relative binding affinities or concentrations. We also delineate advantages of this approach compared to the Cheng-Prusoff and Munson-Rodbard approaches for estimation of Ki using competition binding data.

Human autoreactive T cells recognize CD1b and phospholipids.

In contrast with the common detection of T cells that recognize MHC, CD1a, CD1c, or CD1d proteins, CD1b autoreactive T cells have been difficult to isolate in humans. Here we report the development of polyvalent complexes of CD1b proteins and carbohydrate backbones (dextramers) and their use in identifying CD1b autoreactive T cells from human donors. Activation is mediated by αß T-cell receptors (TCRs) binding to CD1b-phospholipid complexes, which is sufficient to activate autoreactive responses to CD1b-expressing cells. Using mass spectrometry and T-cell responses to scan through the major classes of phospholipids, we identified phosphatidylglycerol (PG) as the immunodominant lipid antigen. T cells did not discriminate the chemical differences that distinguish mammalian PG from bacterial PG. Whereas most models of T-cell recognition emphasize TCR discrimination of differing self and foreign structures, CD1b autoreactive T cells recognize lipids with dual self and foreign origin. PG is rare in the cellular membranes that carry CD1b proteins. However, bacteria and mitochondria are rich in PG, so these data point to a more general mechanism of immune detection of infection- or stress-associated lipids.

Molecular Analysis of Lipid-Reactive Vδ1 γδ T Cells Identified by CD1c Tetramers.

CD1c is abundantly expressed on human dendritic cells (DC) and B cells, where it binds and displays lipid Ags to T cells. In this study, we report that CD1c tetramers carrying Mycobacterium tuberculosis phosphomycoketide bind γδ TCRs. An unbiased method of ligand-based TCR selection detects interactions only with Vδ1(+) TCRs, and mutational analyses demonstrate a role of the Vδ1 domain during recognition. These results strengthen evidence for a role of CD1c in the γδ T cell response, providing biophysical evidence for CD1c-γδ TCR interactions and a named foreign Ag. Surprisingly, TCRs also bind CD1c complexes formed with diverse lipids such as lysophosphatidylcholine, sulfatide, or mannosyl-phosophomycoketide, but not lipopeptide ligands. Dissection of TCR interactions with CD1c carrying foreign Ags, permissive ligands, and nonpermissive lipid ligands clarifies the molecular basis of the frequently observed but poorly understood phenomenon of mixed self- and foreign Ag reactivity in the CD1 system.

Human effector and memory CD8+ T cell responses to smallpox and yellow fever vaccines.

To explore the human T cell response to acute viral infection, we performed a longitudinal analysis of CD8(+) T cells responding to the live yellow fever virus and smallpox vaccines--two highly successful human vaccines. Our results show that both vaccines generated a brisk primary effector CD8(+) T cell response of substantial magnitude that could be readily quantitated with a simple set of four phenotypic markers. Secondly, the vaccine-induced T cell response was highly specific with minimal bystander effects. Thirdly, virus-specific CD8(+) T cells passed through an obligate effector phase, contracted more than 90% and gradually differentiated into long-lived memory cells. Finally, these memory cells were highly functional and underwent a memory differentiation program distinct from that described for human CD8(+) T cells specific for persistent viruses. These results provide a benchmark for CD8(+) T cell responses induced by two of the most effective vaccines ever developed.

Molecular basis of mycobacterial lipid antigen presentation by CD1c and its recognition by αß T cells.

CD1c is a member of the group 1 CD1 family of proteins that are specialized for lipid antigen presentation. Despite high cell surface expression of CD1c on key antigen-presenting cells and the discovery of its mycobacterial lipid antigen presentation capability, the molecular basis of CD1c recognition by T cells is unknown. Here we present a comprehensive functional and molecular analysis of αß T-cell receptor (TCR) recognition of CD1c presenting mycobacterial phosphomycoketide antigens. Our structure of CD1c with the mycobacterial phosphomycoketide (PM) shows similarities to that of CD1c-mannosyl-ß1-phosphomycoketide in that the A' pocket accommodates the mycoketide alkyl chain; however, the phosphate head-group of PM is shifted ∼6 Å in relation to that of mannosyl-ß1-PM. We also demonstrate a bona fide interaction between six human TCRs and CD1c-mycoketide complexes, measuring high to moderate affinities. The crystal structure of the DN6 TCR and mutagenic studies reveal a requirement of five complementarity determining region (CDR) loops for CD1c recognition. Furthermore, mutagenesis of CD1c reveals residues in both the α1 and α2 helices involved in TCR recognition, yet not entirely overlapping among the examined TCRs. Unlike patterns for MHC I, no archetypical binding footprint is predicted to be shared by CD1c-reactive TCRs, even when recognizing the same or similar antigens.

Vaccine-Induced Simian Immunodeficiency Virus-Specific CD8+ T-Cell Responses Focused on a Single Nef Epitope Select for Escape Variants Shortly after Infection.

UNLABELLED: Certain major histocompatibility complex class I (MHC-I) alleles (e.g., HLA-B*27) are enriched among human immunodeficiency virus type 1 (HIV-1)-infected individuals who suppress viremia without treatment (termed "elite controllers" [ECs]). Likewise, Mamu-B*08 expression also predisposes rhesus macaques to control simian immunodeficiency virus (SIV) replication. Given the similarities between Mamu-B*08 and HLA-B*27, SIV-infected Mamu-B*08(+) animals provide a model to investigate HLA-B*27-mediated elite control. We have recently shown that vaccination with three immunodominant Mamu-B*08-restricted epitopes (Vif RL8, Vif RL9, and Nef RL10) increased the incidence of elite control in Mamu-B*08(+) macaques after challenge with the pathogenic SIVmac239 clone. Furthermore, a correlate analysis revealed that CD8(+) T cells targeting Nef RL10 was correlated with improved outcome. Interestingly, this epitope is conserved between SIV and HIV-1 and exhibits a delayed and atypical escape pattern. These features led us to postulate that a monotypic vaccine-induced Nef RL10-specific CD8(+) T-cell response would facilitate the development of elite control in Mamu-B*08(+) animals following repeated intrarectal challenges with SIVmac239. To test this, we vaccinated Mamu-B*08(+) animals with nef inserts in which Nef RL10 was either left intact (group 1) or disrupted by mutations (group 2). Although monkeys in both groups mounted Nef-specific cellular responses, only those in group 1 developed Nef RL10-specific CD8(+) T cells. These vaccine-induced effector memory CD8(+) T cells did not prevent infection. Escape variants emerged rapidly in the group 1 vaccinees, and ultimately, the numbers of ECs were similar in groups 1 and 2. High-frequency vaccine-induced CD8(+) T cells focused on a single conserved epitope and therefore did not prevent infection or increase the incidence of elite control in Mamu-B*08(+) macaques. IMPORTANCE: Since elite control of chronic-phase viremia is a classic example of an effective immune response against HIV/SIV, elucidating the basis of this phenomenon may provide useful insights into how to elicit such responses by vaccination. We have previously established that vaccine-induced CD8(+) T-cell responses against three immunodominant epitopes can increase the incidence of elite control in SIV-infected Mamu-B*08(+) rhesus macaques­a model of HLA-B*27-mediated elite control. Here, we investigated whether a monotypic vaccine-induced CD8(+) T-cell response targeting the conserved "late-escaping" Nef RL10 epitope can increase the incidence of elite control in Mamu-B*08(+) monkeys. Surprisingly, vaccine-induced Nef RL10-specific CD8(+) T cells selected for variants within days after infection and, ultimately, did not facilitate the development of elite control. Elite control is, therefore, likely to involve CD8(+) T-cell responses against more than one epitope. Together, these results underscore the complexity and multidimensional nature of virologic control of lentivirus infection.

Platelets support a protective immune response to LCMV by preventing splenic necrosis.

Severe arenaviral infections in humans are characterized by clinical findings common to other viral hemorrhagic fevers (VHFs), including thrombocytopenia, leukopenia, skin and internal organ hemorrhages, high viral replication, splenic necrosis, and death. Host responses, rather than direct damage by the arenaviral replication, account for most of the observed pathology, but it is not known what protective roles platelets may have in each of the manifestations. To address this issue in an animal model, we compared nondepleted (100%), partially depleted (15%), and profoundly (< 2.5%) platelet depleted mice infected with the mouse arenavirus lymphocytic choriomeningitis virus (LCMV). Here, we describe that systemic bleedings and death were seen only in those animals receiving the stronger depletion treatment. Furthermore, we showed that the nonhemorrhagic but partially platelet-depleted mice were unable to control the viral replication because of generalized splenic necrosis, affecting innate and adaptive immune cells.These data suggest that, by their supportive roles in hemostasis, platelets may be preventing the severe pathology observed in human arenaviral infections.