CD62L expression marks SARS-CoV-2 memory B cell subset with preference for neutralizing epitopes

Severe acute respiratory syndrome coronavirus 2-neutralizing antibodies primarily target the spike receptor binding domain (RBD). However, B cell antigen receptors (BCRs) on RBD-binding memory B (B_mem) cells have variation in the neutralizing activities. Here, by combining single B_mem cell profiling with antibody functional assessment, we dissected the phenotype of B_mem cell harboring the potently neutralizing antibodies in coronavirus disease 2019 (COVID-19)-convalescent individuals. The neutralizing subset was marked by an elevated CD62L expression and characterized by distinct epitope preference and usage of convergent V_H (variable region of immunoglobulin heavy chain) genes, accounting for the neutralizing activities. Concordantly, the correlation was observed between neutralizing antibody titers in blood and CD62L⁺ subset, despite the equivalent RBD binding of CD62L⁺ and CD62L^- subset. Furthermore, the kinetics of CD62L⁺ subset differed between the patients who recovered from different COVID-19 severities. Our B_mem cell profiling reveals the unique phenotype of B_mem cell subset that harbors potently neutralizing BCRs, advancing our understanding of humoral protection.

Heterologous SARS-CoV-2 spike protein booster elicits durable and broad antibody responses against the receptor-binding domain

The immunogenicity of mRNA vaccines has not been well studied when compared to different vaccine modalities in the context of additional boosters. Here we show that longitudinal analysis reveals more sustained SARS-CoV-2 spike receptor-binding domain (RBD)-binding IgG titers with the breadth to antigenically distinct variants by the S-268019-b spike protein booster compared to the BNT162b2 mRNA homologous booster. The durability and breadth of RBD-angiotensin-converting enzyme 2 (ACE2) binding inhibitory antibodies are pronounced in the group without systemic adverse events (AEs) after the S-268019-b booster, leading to the elevated neutralizing activities against Omicron BA.1 and BA.5 variants in the stratified group. In contrast, BNT162b2 homologous booster elicited antibodies to spike N-terminal domain in proportion to the AE scores. High-dimensional immune profiling identifies early CD16⁺ natural killer cell dynamics with CCR3 upregulation, as one of the correlates for the distinct anti-RBD antibody responses by the S-268019-b booster. Our results illustrate the combinational effects of heterologous booster on the immune dynamics and the durability and breadth of recalled anti-RBD antibody responses against emerging virus variants.

XCR1+ DCs are critical for T cell-mediated immunotherapy of chronic viral infections

The contribution of cross-presenting XCR1+ dendritic cells (DCs) and SIRPα+ DCs in maintaining T cell function during exhaustion and immunotherapeutic interventions of chronic infections remains poorly characterized. Using the mouse model of chronic LCMV infection, we found that XCR1+ DCs are more resistant to infection and highly activated compared with SIRPα+ DCs. Exploiting XCR1+ DCs via Flt3L-mediated expansion or XCR1-targeted vaccination notably reinvigorates CD8+ T cells and improves virus control. Upon PD-L1 blockade, XCR1+ DCs are not required for the proliferative burst of progenitor exhausted CD8+ T (TPEX) cells but are indispensable to sustain the functionality of exhausted CD8+ T (TEX) cells. Combining anti-PD-L1 therapy with increased frequency of XCR1+ DCs improves functionality of TPEX and TEX subsets, while increase of SIRPα+ DCs dampened their proliferation. Together, this demonstrates that XCR1+ DCs are crucial for the success of checkpoint inhibitor-based therapies through differential activation of exhausted CD8+ T cell subsets.

SARS-CoV-2-specific CD4+ T cell longevity correlates with Th17-like phenotype

Determinants of memory T cell longevity following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection remain unknown. In addition, phenotypes associated with memory T cell longevity, antibody titers, and disease severity are incompletely understood. Here, we longitudinally analyzed SARS-CoV-2-specific T cell and antibody responses of a unique cohort with similar numbers of mild, moderate, and severe coronavirus disease 2019 cases. The half-lives of CD4⁺ and CD8⁺ T cells were longer than those of antibody titers and showed no clear correlation with disease severity. When CD4⁺ T cells were divided into Th1-, Th2-, Th17-, and Tfh-like subsets, the Th17-like subset showed a longer half-life than other subsets, indicating that Th17-like cells are most closely correlated with T cell longevity. In contrast, Th2- and Tfh-like T cells were more closely correlated with antibody titers than other subsets. These results suggest that distinct CD4⁺ T cell subsets are associated with longevity and antibody responses.

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Th17-like CD4⁺ T cells showed a longer half-life than other CD4⁺ T cell subsets

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Anti-RBD-IgG titers were associated with Th2- and Tfh-like CD4 T cells

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CD45RA+CD8⁺ T cells were correlated with disease severity during the early phase

Distinct immune cell dynamics correlate with the immunogenicity and reactogenicity of SARS-CoV-2 mRNA vaccine

Two doses of Pfizer/BioNTech BNT162b2 mRNA vaccine elicit robust severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-neutralizing antibodies with frequent adverse events. Here, by applying a high-dimensional immune profiling on 92 vaccinees, we identify six vaccine-induced immune dynamics that correlate with the amounts of neutralizing antibodies, the severity of adverse events, or both. The early dynamics of natural killer (NK)/monocyte subsets (CD16+ NK cells, CD56high NK cells, and non-classical monocytes), dendritic cell (DC) subsets (DC3s and CD11c- Axl+ Siglec-6+ [AS]-DCs), and NKT-like cells are revealed as the distinct cell correlates for neutralizing-antibody titers, severity of adverse events, and both, respectively. The cell correlates for neutralizing antibodies or adverse events are consistently associated with elevation of interferon gamma (IFN-γ)-inducible chemokines, but the chemokine receptors CCR2 and CXCR3 are expressed in distinct manners between the two correlates: vaccine-induced expression on the neutralizing-antibody correlate and constitutive expression on the adverse-event correlate. The finding may guide vaccine strategies that balance immunogenicity and reactogenicity.

SARS-CoV-2 Omicron-neutralizing memory B cells are elicited by two doses of BNT162b2 mRNA vaccine

Multiple SARS-CoV-2 variants have mutations in the spike receptor binding domain (RBD) with potential to evade neutralizing antibody. In particular, the Beta and Omicron variants escape from antibody neutralizing activity in those who received two doses of BNT162b2 mRNA vaccine. Nonetheless, boosting with a third vaccine dose or by breakthrough infection improves the overall breadth of the neutralizing antibodies, but the mechanism remains unclear. Here, we longitudinally profiled the cellular composition of RBD-binding memory B cell subsets and their antibody binding and neutralizing activity against SARS-CoV-2 variants after the second dose of mRNA vaccine. Two doses of the mRNA vaccine elicited plasma neutralizing antibodies with a limited activity against Beta and Omicron but induced an expanded antibody breadth overtime, up to 4.9 months after vaccination. In contrast, more than one-third of RBD-binding IgG+ memory B cells with a resting phenotype initially bound the Beta and Omicron variants and steadily increased the B cell receptor breadth overtime. As a result, a fraction of the resting memory B cell subset secreted Beta and Omicron-neutralizing antibody when stimulated in vitro. The neutralizing breadth of the resting memory B cell subset helps us understand the prominent recall of Omicron-neutralizing antibodies after an additional booster or breakthrough infection in fully vaccinated individuals.

Env-independent protection of intrarectal SIV challenge by vaccine induction of Gag/Vif-specific CD8+ T cells but not CD4+ T cells

Effective T cell induction is an important strategy in HIV-vaccine development. However, it has been indicated that vaccine-induced HIV-specific CD4⁺ T cells, the preferential targets of HIV infection, might increase viral acquisition after HIV exposure. We have recently developed an immunogen (CaV11), tandemly connected overlapping 11-mer peptides spanning the simian immunodeficiency virus (SIV) Gag capsid and Vif proteins, to selectively induce Gag- and Vif-specific CD8⁺ T cells but not CD4⁺ T cells. Here, we show protective efficacy of a CaV11-expressing vaccine against repeated intrarectal low-dose SIVmac239 challenge in rhesus macaques. Eight of the twelve vaccinated macaques were protected after eight challenges. Kaplan-Meier analysis indicated significant protection in the vaccinees compared to the unvaccinated macaques. Vaccine-induced Gag-specific CD8⁺ T cell responses were significantly higher in the protected than the unprotected vaccinees. These results suggest that classical CD8⁺ T cell induction by viral Env-independent vaccination can confer protection from intrarectal SIV acquisition, highlighting the rationale for this immunogen design to induce virus-specific CD8⁺ T cells but not CD4⁺ T cells in HIV-vaccine development.

Temporal maturation of neutralizing antibodies in COVID-19 convalescent individuals improves potency and breadth to circulating SARS-CoV-2 variants

Antibody titers against SARS-CoV-2 slowly wane over time. Here, we examined how time affects antibody potency. To assess the impact of antibody maturation on durable neutralizing activity against original SARS-CoV-2 and emerging variants of concern (VOCs), we analyzed receptor binding domain (RBD)-specific IgG antibodies in convalescent plasma taken 1-10 months after SARS-CoV-2 infection. Longitudinal evaluation of total RBD IgG and neutralizing antibody revealed declining total antibody titers but improved neutralization potency per antibody to original SARS-CoV-2, indicative of antibody response maturation. Neutralization assays with authentic viruses revealed that early antibodies capable of neutralizing original SARS-CoV-2 had limited reactivity toward B.1.351 (501Y.V2) and P.1 (501Y.V3) variants. Antibodies from late convalescents exhibited increased neutralization potency to VOCs, suggesting persistence of cross-neutralizing antibodies in plasma. Thus, maturation of the antibody response to SARS-CoV-2 potentiates cross-neutralizing ability to circulating variants, suggesting that declining antibody titers may not be indicative of declining protection.

Perspectives on Non-BLT Humanized Mouse Models for Studying HIV Pathogenesis and Therapy

A variety of humanized mice, which are reconstituted only with human hematopoietic stem cells (HSC) or with fetal thymus and HSCs, have been developed and widely utilized as in vivo animal models of HIV-1 infection. The models represent some aspects of HIV-mediated pathogenesis in humans and are useful for the evaluation of therapeutic regimens. However, there are several limitations in these models, including their incomplete immune responses and poor distribution of human cells to the secondary lymphoid tissues. These limitations are common in many humanized mouse models and are critical issues that need to be addressed. As distinct defects exist in each model, we need to be cautious about the experimental design and interpretation of the outcomes obtained using humanized mice. Considering this point, we mainly characterize the current conventional humanized mouse reconstituted only with HSCs and describe past achievements in this area, as well as the potential contributions of the humanized mouse models for the study of HIV pathogenesis and therapy. We also discuss the use of various technologies to solve the current problems. Humanized mice will contribute not only to the pre-clinical evaluation of anti-HIV regimens, but also to a deeper understanding of basic aspects of HIV biology.

Association of HLA-DRB1*09:01 with severe COVID-19

HLA‐A, ‐C, ‐B, and ‐DRB1 genotypes were analyzed in 178 Japanese COVID‐19 patients to investigate the association of HLA with severe COVID‐19. Analysis of 32 common HLA alleles at four loci revealed a significant association between HLA‐DRB1*09:01 and severe COVID‐19 (odds ratio [OR], 3.62; 95% CI, 1.57–8.35; p = 0.00251 [permutation p value = 0.0418]) when age, sex, and other common HLA alleles at the DRB1 locus were adjusted. The DRB1*09:01 allele was more significantly associated with risk for severe COVID‐19 compared to preexisting medical conditions such as hypertension, diabetes, and cardiovascular diseases. These results indicate a potential role for HLA in predisposition to severe COVID‐19.

Myeloid cell dynamics correlating with clinical outcomes of severe COVID-19 in Japan

An expanded myeloid cell compartment is a hallmark of severe coronavirus disease 2019 (COVID-19). However, data regarding myeloid cell expansion have been collected in Europe, where the mortality rate by COVID-19 is greater than those in other regions including Japan. Thus, characteristics of COVID-19-induced myeloid cell subsets remain largely unknown in the regions with low mortality rates. Here, we analyzed cellular dynamics of myeloid-derived suppressor cell (MDSC) subsets and examined whether any of them correlate with disease severity and prognosis, using blood samples from Japanese COVID-19 patients. We observed that polymorphonuclear (PMN)-MDSCs, but not other MDSC subsets, transiently expanded in severe cases but not in mild or moderate cases. Contrary to previous studies in Europe, this subset selectively expanded in survivors of severe cases and subsided before discharge, but such transient expansion was not observed in non-survivors in Japanese cohort. Analysis of plasma cytokine/chemokine levels revealed positive correlation of PMN-MDSC frequencies with interleukin 8 (IL-8) levels prior to the cell expansion, indicating the involvement of IL-8 on recruitment of PMN-MDSCs to peripheral blood following the onset of severe COVID-19. Thus, our data indicates that transient expansion of the PMN-MDSC subset results in improved clinical outcome. Thus, this myeloid cell subset may be a predictor of prognosis in cases of severe COVID-19 in Japan.

Development of an Inflammatory CD14+ Dendritic Cell Subset in Humanized Mice

Humanized mouse models are attractive experimental models for analyzing the development and functions of human dendritic cells (DCs) in vivo. Although various types of DC subsets, including DC type 3 (DC3s), have been identified in humans, it remains unclear whether humanized mice can reproduce heterogeneous DC subsets. CD14, classically known as a monocyte/macrophage marker, is reported as an indicator of DC3s. We previously observed that some CD14⁺ myeloid cells expressed CD1c, a pan marker for bona fide conventional DC2 (cDC2s), in humanized mouse models in which human FLT3L and GM-CSF genes were transiently expressed using in vivo transfection (IVT). Here, we aimed to elucidate the identity of CD14⁺CD1c⁺ DC-like cells in humanized mouse models. We found that CD14⁺CD1c⁺ cells were phenotypically different from cDC2s; CD14⁺CD1c⁺ cells expressed CD163 but not CD5, whereas cDC2s expressed CD5 but not CD163. Furthermore, CD14⁺CD1c⁺ cells primed and polarized naïve CD4⁺ T cells toward IFN-γ⁺ Th1 cells more profoundly than cDC2s. Transcriptional analysis revealed that CD14⁺CD1c⁺ cells expressed several DC3-specific transcripts, such as CD163, S100A8, and S100A9, and were clearly segregated from cDC2s and monocytes. When lipopolysaccharide was administered to the humanized mice, the frequency of CD14⁺CD1c⁺ cells producing IL-6 and TNF-α was elevated, indicating a pro-inflammatory signature. Thus, humanized mice are able to sustain development of functional CD14⁺CD1c⁺ DCs, which are equivalent to DC3 subset observed in humans, and they could be useful for analyzing the development and function of DC3s in vivo.

Substantial induction of non-apoptotic CD4 T-cell death during the early phase of HIV-1 infection in a humanized mouse model

Several mechanisms underline induction of CD4 T-cell death by human immunodeficiency virus (HIV) infection. For a long time, apoptosis was considered central to cell death involved in the depletion of CD4 T cells during HIV infection. However, which types of cell death are induced during the early phase of HIV infection in vivo remains unclear. In this study, CD4 T-cell death induced in early HIV infection was characterized using humanized mice challenged with CCR5-tropic (R5) or CXCR4-tropic (X4) HIV-1. Results showed that CD4 T-cell death was induced in the spleen 3 days post-challenge with both R5 and X4 HIV-1. Although cell death without caspase-1 and caspase-3/7 activation was preferentially observed, caspase-1⁺ pyroptosis was also significantly induced within the memory subpopulation by R5 or X4 HIV-1 and the naïve subpopulation by X4 HIV-1. In contrast, caspase-3/7⁺ apoptosis was not enhanced by either R5 or X4 HIV-1. Furthermore, phosphorylated mixed lineage kinase domain-like protein⁺ necroptosis was induced by only X4 HIV-1. These findings indicate that various types of non-apoptotic CD4 T-cell death, such as pyroptosis and necroptosis, are induced during the early phase of HIV infection in vivo.

CD4+ T Responses Other Than Th1 Type Are Preferentially Induced by Latency-Associated Antigens in the State of Latent Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis (M. tuberculosis) produces a diverse range of antigenic proteins in its dormant phase. The cytokine profiles of CD4⁺ T cell responses, especially subsets other than Th1 type (non-Th1 type), against these latency-associated M. tuberculosis antigens such as α-crystallin (Acr), heparin-binding hemagglutinin (HBHA), and mycobacterial DNA-binding protein 1 (MDP-1) remain elusive in relation to the clinical stage of M. tuberculosis infection. In the present study, peripheral blood mononuclear cells (PBMCs) collected from different stages of M. tuberculosis-infected cases and control PBMCs were stimulated with these antigens and ESAT-6/CFP-10. Cytokine profiles of CD4⁺ T cells were evaluated by intracellular cytokine staining using multicolor flow cytometry. Our results demonstrate that Th1 cytokine responses were predominant after TB onset independent of the type of antigen stimulation. On the contrary, non-Th1 cytokine responses were preferentially induced by latency-associated M. tuberculosis antigens, specifically IL-10 response against Acr in latent M. tuberculosis infection. From these results, we surmise a shift in the CD4⁺ T cell response from mixed non-Th1 to Th1 dominant type during TB progression.

Publisher Correction: A humanized mouse model identifies key amino acids for low immunogenicity of H7N9 vaccines

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A CCR5+ memory subset within HIV-1-infected primary resting CD4+ T cells is permissive for replication-competent, latently infected viruses in vitro

OBJECTIVE

Resting CD4⁺ T cells are major reservoirs of latent HIV-1 infection, and may be formed during the early phase of the infection. Although CCR5-tropic (R5) HIV-1 is highly transmissible during the early phase, newly infected individuals have usually been exposed to a mixture of R5 and CXCR4-tropic (X4) viruses, and X4 viral DNA is also detectable in the host. Our aim was to identify which subsets of resting CD4⁺ T cells contribute to forming the latent reservoir in the presence of both X4 and R5 viruses.

RESULTS

Primary resting CD4⁺ naïve T (T_N) cells, CCR5^- memory T (T_M) cells, and CCR5⁺ T_M cells isolated by flow cytometry were infected simultaneously with X4 and R5 HIV-1, which harbored different reporter genes, and were cultured in the resting condition. Flow cytometry at 3 days post-infection demonstrated that X4 HIV-1⁺ cells were present in all three subsets of cells, whereas R5 HIV-1⁺ cells were present preferentially in CCR5⁺ T_M cells, but not in T_N cells. Following CD3/CD28-mediated activation at 3 days post-infection, numbers of R5 HIV-1⁺ cells and X4 HIV-1⁺ cells increased significantly only in the CCR5⁺ T_M subset, suggesting that it provides a major reservoir of replication-competent, latently infected viruses.

STING agonists activate latently infected cells and enhance SIV-specific responses ex vivo in naturally SIV controlled cynomolgus macaques

To achieve a functional cure for HIV, treatment regimens that eradicate latently HIV-infected cells must be established. For this, many groups have attempted to reactivate latently-infected cells to induce cytopathic effects and/or elicit cytotoxic T lymphocyte (CTL)/NK cell-mediated immune responses to kill these cells. We believe that not only the reactivation of latently-infected cells, but also the induction of strong CTL responses, would be required for this. Here, we used typical immune activators that target pattern recognition receptors (PRRs). For our experimental model, we identified eight SIV-infected cynomolgus monkeys that became natural controllers of viremia. Although plasma viral loads were undetectable, we could measure SIV-DNA by qPCR in peripheral blood mononuclear cells (PBMCs). Using these PBMCs, we screened 10 distinct PRR ligands to measure IFN-α and IFN-γ production. Among these, STING ligands, cGAMP and c-di-AMP, and the TLR7/8 agonist R848 markedly increased cytokine levels. Both R848 and STING ligands could reactivate latently-infected cells in both cynomolgus monkeys and human PBMCs in vitro. Furthermore, c-di-AMP increased the frequency of SIV Gag-specific CD8⁺ T cells including polyfunctional CD8⁺ T cells, as compared to that in untreated control or R848-treated cells. Together, STING ligands might be candidates for HIV treatment.

Introduction of Human Flt3-L and GM-CSF into Humanized Mice Enhances the Reconstitution and Maturation of Myeloid Dendritic Cells and the Development of Foxp3+CD4+ T Cells

Two cytokines, fms-related tyrosine kinase 3 ligand (Flt3-L) and granulocyte-macrophage colony-stimulating factor (GM-CSF) are considered to be the essential regulators of dendritic cell (DC) development in vivo. However, the combined effect of Flt3-L and GM-CSF on human DCs has not been evaluated in vivo. In this study, we, therefore, aimed at evaluating this using a humanized mouse model. Humanized non-obese diabetic/SCID/Jak3^null (hNOJ) mice were constructed by transplanting hematopoietic stem cells from human umbilical cord blood into newborn NOJ mice, and in vivo transfection (IVT) was performed by hydrodynamic injection-mediated gene delivery using plasmids encoding human Flt3-L and GM-CSF. Following IVT, Flt3-L and GM-CSF were successfully induced in hNOJ mice. At 10 days post-IVT, we found, in the spleen, that treatment with both Flt3-L and GM-CSF enhanced the reconstitution of two myeloid DC subsets, CD14⁻CD1c⁺ conventional DCs (cDCs) and CD14⁻CD141⁺ cDCs, in addition to CD14⁺ monocyte-like cells expressing CD1c and/or CD141. GM-CSF alone had less effect on the reconstitution of these myeloid cell populations. By contrast, none of the cytokine treatments enhanced CD123⁺ plasmacytoid DC (pDC) reconstitution. Regardless of the reconstitution levels, three cell populations (CD1c⁺ myeloid cells, CD141⁺ myeloid cells, and pDCs) could be matured by treatment with cytokines, in terms of upregulation of CD40, CD80, CD86, and CD184/CXCR4 and downregulation of CD195/CCR5. In particular, GM-CSF contributed to upregulation of CD80 in all these cell populations. Interestingly, we further observed that Foxp3⁺ cells within splenic CD4⁺ T cells were significantly increased in the presence of GM-CSF. Foxp3⁺ T cells could be subdivided into two subpopulations, CD45RA⁻Foxp3^hi and CD45RA⁻Foxp3^lo T cells. Whereas CD45RA⁻Foxp3^hi T cells were increased only after treatment with GM-CSF alone, CD45RA⁻Foxp3^lo T cells were increased only after treatment with both Flt3-L and GM-CSF. Treatment with Flt3-L alone had no effect on the number of Foxp3⁺ T cells. The correlation analysis demonstrated that the development of these Foxp3⁺ subpopulations was associated with the maturation status of DC(-like) cells. Taken together, this study provides a platform for studying the in vivo effect of Flt3-L and GM-CSF on human DCs and regulatory T cells.

HIV LTR-Driven Antisense RNA by Itself Has Regulatory Function and May Curtail Virus Reactivation From Latency

Latently infected T lymphocytes are an important barrier toward eliminating a persistent HIV infection. Here we describe an HIV-based recombinant fluorescent-lentivirus referred to as “rfl-HIV” that enables to analyze sense and antisense transcription by means of fluorescence reporter genes. This model virus exhibited similar transcriptional and functional properties of the antisense transcript as observed with a wild type HIV, and largely facilitated the generation of latently-infected T cells clones. We show that latently-infected cells can be divided into two types, those with and those without antisense transcription. Upon addition of latency reversal agents, only the cells that lack antisense transcripts are readily reactivated to transcribe HIV. Thus, antisense transcripts may exhibit a dominant suppressor activity and can lock an integrated provirus into a non-reactivatable state. These findings could have important implications for the development of strategies to eradicate HIV from infected individuals.

Homeostatically Maintained Resting Naive CD4+ T Cells Resist Latent HIV Reactivation

Homeostatic proliferation (HSP) is a major mechanism by which long-lived naïve and memory CD4⁺ T cells are maintained in vivo and suggested to contribute to the persistence of the latent HIV-1 reservoir. However, while many in vitro latency models rely on CD4⁺ T cells that were initially differentiated via T-cell receptor (TCR) stimulation into memory/effector cells, latent infection of naïve resting CD4⁺ T cells maintained under HSP conditions has not been fully addressed. Here, we describe an in vitro HSP culture system utilizing the cytokines IL-7 and IL-15 that allows studying latency in naïve resting CD4⁺ T cells. CD4⁺ T cells isolated from several healthy donors were infected with HIV pseudotypes expressing GFP and cultured under HSP conditions or TCR conditions as control. Cell proliferation, phenotype, and GFP expression were analyzed by flow cytometry. RNA expression was quantified by qRT-PCR. Under HSP culture conditions, latently HIV-1 infected naïve cells are in part maintained in the non-dividing (= resting) state. Although a few HIV-1 provirus⁺ cells were present in these resting GFP negative cells, the estimated level of GFP transcripts per infected cell seems to indicate a block at the post-transcriptional level. Interestingly, neither TCR nor the prototypic HDAC inhibitor SAHA were able to reactivate HIV-1 provirus from these cells. This lack of reactivation was not due to methylation of the HIV LTR. These results point to a mechanism of HIV control in HSP-cultured resting naïve CD4⁺ T cells that may be distinct from that in TCR-stimulated memory/effector T cells.

A novel dual luciferase assay for the simultaneous monitoring of HIV infection and cell viability

Human immunodeficiency virus type 1 (HIV-1) reporter cell lines are critical tools for drug development. However, one disadvantage of HIV-1 reporter cell lines is that reductions in reporter gene activity need to be normalized to cytotoxicity, i.e., live cell numbers. Here, we developed a dual luciferase assay based on a R. reniformis luciferase (hRLuc)-expressing R5-type HIV-1 (NLAD8-hRLuc) and a CEM cell line expressing CCR5 and firefly luciferase (R5CEM-FiLuc). The NLAD8-hRLuc reporter virus was replication competent in peripheral blood mononuclear cells. The level of hRLuc was correlated with p24 antigen levels (p<0.001, R=0.862). The target cell line, R5CEM-FiLuc, stably expressed the firefly luciferase (FiLuc) reporter gene and allowed the simultaneous monitoring of compound cytotoxicity. The dual reporter assay combining a NLAD8-hRLuc virus with R5CEM-FiLuc cells permitted the accurate determination of drug susceptibility for entry, reverse transcriptase, integrase, and protease inhibitors at different multiplicities of infection. This dual reporter assay provides a rapid and direct method for the simultaneous monitoring of HIV infection and cell viability.

Humanized mice dually challenged with R5 and X4 HIV-1 show preferential R5 viremia and restricted X4 infection of CCR5(+)CD4(+) T cells

CCR5-tropic (R5) immunodeficiency virus type 1 (HIV-1) strains are highly transmissible during the early stage of infection in humans, whereas CXCR4-tropic (X4) strains are less transmissible. This study aimed to explore the basis for early phase R5 and X4 HIV-1 infection in vivo by using humanized mice dually challenged with R5 HIV-1NLAD8-D harboring DsRed and X4 HIV-1(NL-E) harboring EGFP. Whereas R5 HIV-1 replicated well, X4 HIV-1 caused only transient viremia with variable kinetics; however, this was distinct from the low level but persistent viremia observed in mice challenged with X4 HIV-1 alone. Flow cytometric analysis of HIV-1-infected cells revealed that X4 HIV-1 infection of CCR5(+)CD4(+) T cells was significantly suppressed in the presence of R5 HIV-1. X4 HIV-1 was more cytopathic than R5 HIV-1; however, this was not the cause of restricted X4 HIV-1 infection because there were no significant differences in the mortality rates of CCR5(+) and CCR5(-) cells within the X4 HIV-1-infected cell populations. Taken together, these results suggest that restricted infection of CCR5(+)CD4(+) T cells by X4 HIV-1 (occurring via a still-to-be-identified mechanism) might contribute to the preferential transmission of R5 HIV-1 during the early phase of infection.

Broad cross-reactive epitopes of the H5N1 influenza virus identified by murine antibodies against the A/Vietnam/1194/2004 hemagglutinin

There is an urgent need for a rapid diagnostic system to detect the H5 subtype of the influenza A virus. We previously developed monoclonal antibodies (mAbs) against the H5 hemagglutinin (HA) for use in a rapid diagnostic kit. In this study, we determined the epitopes of the anti-H5 HA murine mAbs OM-b, AY-2C2, and YH-1A1. Binding assays of the mAbs to different strains of H5 HAs indicated that OM-b and AY-2C2 cross-reacted with HAs from clades 1, 2.1.3.2, 2.2, and 2.3.4, whereas YH-1A1 failed to bind to those of clades 2.1.3.2 and 2.3.4. HA chimeras revealed that the epitopes for each of the mAbs were in the HA1 region. Analysis of escape mutants revealed that OM-b and AY-2C2 mAbs interacted mainly with amino acid residues D43 and G46, and the YH-1A1 mAb interacted with G139 and K or R140 of H5 HA. Multiple alignments of H5 HA protein sequences showed that D43 and G46 were very conserved among H5N1 HAs, except those in clade 2.2.1 and clade 7 (88.7%). The epitope for YH-1A1 mAb was highly variable in the HAs of H5N1, although it was well conserved in those of H5N2-N9. The OM-b and AY-2C2 mAbs could bind to the HAs of clades 1.1 and 2.3.2.1 that are currently epidemic in Asia, and we conclude that these would be effective for the detection of H5N1 infections in this region.

Sensitive detection of measles virus infection in the blood and tissues of humanized mouse by one-step quantitative RT-PCR

Live attenuated measles virus (MV) has long been recognized as a safe and effective vaccine, and it has served as the basis for development of various MV-based vaccines. However, because MV is a human-tropic virus, the evaluation of MV-based vaccines has been hampered by the lack of a small-animal model. The humanized mouse, a recently developed system in which an immunodeficient mouse is transplanted with human fetal tissues or hematopoietic stem cells, may represent a suitable model. Here, we developed a sensitive one-step quantitative reverse transcription (qRT)-PCR that simultaneously measures nucleocapsid (N) and human RNase P mRNA levels. The results can be used to monitor MV infection in a humanized mouse model. Using this method, we elucidated the replication kinetics of MV expressing enhanced green fluorescent protein both in vitro and in humanized mice in parallel with flow-cytometric analysis. Because our qRT-PCR system was sensitive enough to detect MV expression using RNA extracted from a small number of cells, it can be used to monitor MV infection in humanized mice by sequential blood sampling.

Expansion of activated memory CD4+ T cells affects infectivity of CCR5-tropic HIV-1 in humanized NOD/SCID/JAK3null mice

Humanized mice reconstituted with human hematopoietic cells have been developed as an experimental animal model for human immunodeficiency virus type 1 (HIV-1) infection. Myeloablative irradiation is usually performed to augment the engraftment of donor hematopoietic stem cells (HSCs) in recipient mice; however, some mouse strains are susceptible to irradiation, making longitudinal analysis difficult. We previously attempted to construct humanized NOD/SCID/JAK3^null (hNOJ) mice, which were not irradiated prior to human HSC transplantation. We found that, over time, many of the reconstituted CD4⁺ T cells expanded with an activated effector memory phenotype. Therefore, the present study used hNOJ mice that were irradiated (hNOJ (IR+)) or not (hNOJ (IR−)) prior to human HSC transplantation to examine whether the development and cellularity of the reconstituted CD4⁺ T cells were influenced by the degree of chimerism, and whether they affected HIV-1 infectivity. Indeed, hNOJ (IR+) mice showed a greater degree of chimerism than hNOJ (IR−) mice. However, the conversion of CD4⁺ T cells to an activated effector memory phenotype, with a high percentage of cells showing Ki-67 expression, occurred in both hNOJ (IR+) and hNOJ (IR−) mice, probably as a result of lymphopenia-induced homeostatic expansion. Furthermore, when hNOJ (IR+) and hNOJ (IR−) mice, which were selected as naïve- and memory CD4⁺ T cell subset-rich groups, respectively, were infected with CCR5-tropic HIV-1 in vivo, virus replication (as assessed by the plasma viral load) was delayed; however, the titer subsequently reached a 1-log higher level in memory-rich hNOJ (IR−) mice than in naïve-rich hNOJ (IR+) mice, indicating that virus infectivity in hNOJ mice was affected by the different status of the reconstituted CD4⁺ T cells. Therefore, the hNOJ mouse model should be used selectively, i.e., according to the specific experimental objectives, to gain an appropriate understanding of HIV-1 infection/pathogenesis.

Fluorescent Reporter Signals, EGFP, and DsRed, Encoded in HIV-1 Facilitate the Detection of Productively Infected Cells and Cell-Associated Viral Replication Levels

Flow cytometric analysis is a reliable and convenient method for investigating molecules at the single cell level. Previously, recombinant human immunodeficiency virus type 1 (HIV-1) strains were constructed that express a fluorescent reporter, either enhanced green fluorescent protein, or DsRed, which allow the monitoring of HIV-1-infected cells by flow cytometry. The present study further investigated the potential of these recombinant viruses in terms of whether the HIV-1 fluorescent reporters would be helpful in evaluating viral replication based on fluorescence intensity. When primary CD4⁺ T cells were infected with recombinant viruses, the fluorescent reporter intensity measured by flow cytometry was associated with the level of CD4 downmodulation and Gag p24 expression in infected cells. Interestingly, some HIV-1-infected cells, in which CD4 was only moderately downmodulated, were reporter-positive but Gag p24-negative. Furthermore, when the activation status of primary CD4⁺ T cells was modulated by T cell receptor-mediated stimulation, we confirmed the preferential viral production upon strong stimulation and showed that the intensity of the fluorescent reporter within a proportion of HIV-1-infected cells was correlated with the viral replication level. These findings indicate that a fluorescent reporter encoded within HIV-1 is useful for the sensitive detection of productively infected cells at different stages of infection and for evaluating cell-associated viral replication at the single cell level.

Distinct fucosylation of M cells and epithelial cells by Fut1 and Fut2, respectively, in response to intestinal environmental stress

The intestinal epithelium contains columnar epithelial cells (ECs) and M cells, and fucosylation of the apical surface of ECs and M cells is involved in distinguishing the two populations and in their response to commensal flora and environmental stress. Here, we show that fucosylated ECs (F-ECs) were induced in the mouse small intestine by the pro-inflammatory agents dextran sodium sulfate and indomethacin, in addition to an enteropathogen derived cholera toxin. Although F-ECs showed specificity for the M cell-markers, lectin Ulex europaeus agglutinin-1 and our monoclonal antibody NKM 16-2-4, these cells also retained EC-phenotypes including an affinity for the EC-marker lectin wheat germ agglutinin. Interestingly, fucosylation of Peyer's patch M cells and F-ECs was distinctly regulated by α(1,2)fucosyltransferase Fut1 and Fut2, respectively. These results indicate that Fut2-mediated F-ECs share M cell-related fucosylated molecules but maintain distinctive EC characteristics, Fut1 is, therefore, a reliable marker for M cells.

Selective transmission of R5 HIV-1 over X4 HIV-1 at the dendritic cell-T cell infectious synapse is determined by the T cell activation state

Dendritic cells (DCs) are essential antigen-presenting cells for the induction of T cell immunity against HIV. On the other hand, due to the susceptibility of DCs to HIV infection, virus replication is strongly enhanced in DC–T cell interaction via an immunological synapse formed during the antigen presentation process. When HIV-1 is isolated from individuals newly infected with the mixture of R5 and X4 variants, R5 is predominant, irrespective of the route of infection. Because the early massive HIV-1 replication occurs in activated T cells and such T-cell activation is induced by antigen presentation, we postulated that the selective expansion of R5 may largely occur at the level of DC–T cell interaction. Thus, the immunological synapse serves as an infectious synapse through which the virus can be disseminated in vivo. We used fluorescent recombinant X4 and R5 HIV-1 consisting of a common HIV-1 genome structure with distinct envelopes, which allowed us to discriminate the HIV-1 transmitted from DCs infected with the two virus mixtures to antigen-specific CD4⁺ T cells by flow cytometry. We clearly show that the selective expansion of R5 over X4 HIV-1 did occur, which was determined at an early entry step by the activation status of the CD4⁺ T cells receiving virus from DCs, but not by virus entry efficiency or productivity in DCs. Our results imply a promising strategy for the efficient control of HIV infection.

The cellular tropism of HIV-1 is determined by the binding of HIV-1 envelope to chemokine coreceptors, CCR5 or CXCR4, in addition to a major entry receptor, CD4. The mystery still now is that despite the mixed infection of CCR5-utilizing (R5) and CXCR4-utilizing (X4) HIV-1 in many AIDS patients, R5 is predominantly isolated from newly infected individuals whatever the mode of infection. Because the early massive HIV-1 replication occurs in activated T cells and such T-cell activation is induced initially by antigen-presenting DCs, we postulated that the selective expansion of R5 may largely occur at the level of antigen-dependent DC–T cell interaction, called immunological synapse. Thus, the immunological synapse serves as an infectious synapse through which the virus can be rapidly disseminated in vivo. In this study, we prepared X4 and R5 HIV-1 expressing red or green fluorescence and showed that the selective expansion of R5 over X4 did occur, depending on the activation status of CD4⁺ T cells receiving virus from DCs, but not by virus entry efficiency or productivity in DCs.

Comprehensive gene expression profiling of Peyer's patch M cells, villous M-like cells, and intestinal epithelial cells

Separate populations of M cells have been detected in the follicle-associated epithelium of Peyer's patches (PPs) and the villous epithelium of the small intestine, but the traits shared by or distinguishing the two populations have not been characterized. Our separate study has demonstrated that a potent mucosal modulator cholera toxin (CT) can induce lectin Ulex europaeus agglutinin-1 and our newly developed M cell-specific mAb NKM 16-2-4-positive M-like cells in the duodenal villous epithelium. In this study, we determined the gene expression of PP M cells, CT-induced villous M-like cells, and intestinal epithelial cells isolated by a novel approach using FACS. Additional mRNA and protein analyses confirmed the specific expression of glycoprotein 2 and myristoylated alanine-rich C kinase substrate (MARCKS)-like protein by PP M cells but not CT-induced villous M-like cells. Comprehensive gene profiling also suggested that CT-induced villous M-like cells share traits of both PP M cells and intestinal epithelial cells, a finding that is supported by their unique expression of specific chemokines. The genome-wide assessment of gene expression facilitates discovery of M cell-specific molecules and enhances the molecular understanding of M cell immunobiology.

A single amino acid substitution in the S1 and S2 Spike protein domains determines the neutralization escape phenotype of SARS-CoV

In response to SARS-CoV infection, neutralizing antibodies are generated against the Spike (S) protein. Determination of the active regions that allow viral escape from neutralization would enable the use of these antibodies for future passive immunotherapy. We immunized mice with UV-inactivated SARS-CoV to generate three anti-S monoclonal antibodies, and established several neutralization escape mutants with S protein. We identified several amino acid substitutions, including Y442F and V601G in the S1 domain and D757N and A834V in the S2 region. In the presence of each neutralizing antibody, double mutants with substitutions in both domains exhibited a greater growth advantage than those with only one substitution. Importantly, combining two monoclonal antibodies that target different epitopes effected almost complete suppression of wild type virus replication. Thus, for effective passive immunotherapy, it is important to use neutralizing antibodies that recognize both the S1 and S2 regions.

A novel M cell-specific carbohydrate-targeted mucosal vaccine effectively induces antigen-specific immune responses

Mucosally ingested and inhaled antigens are taken up by membranous or microfold cells (M cells) in the follicle-associated epithelium of Peyer's patches or nasopharynx-associated lymphoid tissue. We established a novel M cell–specific monoclonal antibody (mAb NKM 16–2-4) as a carrier for M cell–targeted mucosal vaccine. mAb NKM 16–2-4 also reacted with the recently discovered villous M cells, but not with epithelial cells or goblet cells. Oral administration of tetanus toxoid (TT)– or botulinum toxoid (BT)–conjugated NKM 16–2-4, together with the mucosal adjuvant cholera toxin, induced high-level, antigen-specific serum immunoglobulin (Ig) G and mucosal IgA responses. In addition, an oral vaccine formulation of BT-conjugated NKM 16–2-4 induced protective immunity against lethal challenge with botulinum toxin. An epitope analysis of NKM 16–2-4 revealed specificity to an α(1,2)-fucose–containing carbohydrate moiety, and reactivity was enhanced under sialic acid–lacking conditions. This suggests that NKM 16–2-4 distinguishes α(1,2)-fucosylated M cells from goblet cells containing abundant sialic acids neighboring the α(1,2) fucose moiety and from non-α(1,2)-fucosylated epithelial cells. The use of NKM 16–2-4 to target vaccine antigens to the M cell–specific carbohydrate moiety is a new strategy for developing highly effective mucosal vaccines.

Differences in integrin-dependent phagocytosis among three hemocyte subpopulations of the Pacific oyster "Crassostrea gigas"

Integrins play a key role in immunoresponses such as attachment, spreading, and phagocytosis in invertebrate hemocytes. This study was designed to identify integrin expression patterns at the hemocyte subpopulation level, and correlate the expression levels with phagocytic ability. First, we cloned a beta integrin from Crassostreagigas hemocytes and used real-time RT-PCR to analyze the quantitative expression level of its encoding mRNA. The expression level in hyalinocytes was significantly higher than that in granulocytes and agranulocytes. Subsequently, we investigated the phagocytic ability of each subpopulation using anti-alpha(5)beta(1) integrin antibody, and found that phagocytosis of hyalinocytes was inhibited by neutralization with the antibody but enhanced against the antibody-conjugated microspheres. In contrast, phagocytic abilities of granulocytes and agranulocytes showed high and zero levels, respectively, regardless of the antibody. These results suggest that phagocytosis of hyalinocytes is regulated by an integrin-dependent mechanism and that of granulocytes is elicited by other functional receptors.

Pacific oyster hemocytes undergo apoptosis following cell-adhesion mediated by integrin-like molecules

Previously, we demonstrated that in the Pacific oyster (Crassostrea gigas, a bivalve mollusc) apoptosis could be induced in hemocytes by treatment with Arg-Gly-Asp (RGD) peptides which are known to function as an integrin ligand. However, it is unclear where the RGD peptides are binding to the C. gigas hemocytes, or what mechanism or molecules are involved, e.g., integrin-ligand interactions. Therefore, this study was undertaken to investigate the binding interactions in C. gigas hemocytes. Initially, to confirm the presence of RGD-recognizing integrin-like molecule(s) on the hemocytes, we assessed the enhancement of spreading ability, and found that spreading ability was enhanced by immobilized human fibronectin, a fibronectin fragment containing the RGD motif, and C. gigas plasma in the presence of divalent cations. Interestingly, viability of the spreading hemocytes dramatically decreased 24 h later and DNA fragmentation with oligonucleosomal laddering of 180-200 bp in length was detected in the dead hemocytes by electrophoresis and TUNEL assay. These results indicated that hemocyte adhesion mediated by integrin-like molecules triggered apoptosis and suggested that integrin-activation contributes to the induction of apoptosis. This is the first report showing the possibility of an integrin functioning in the induction of apoptosis in invertebrate hemocytes.

Biological role of Ep-CAM in the physical interaction between epithelial cells and lymphocytes in intestinal epithelium

The mucosal epithelium including intestinal epithelial cells (IECs) and intraepithelial lymphocytes (IELs) provide a first line of defense in the gastrointestinal tract. However, limited information is currently available concerning the nature of the physical interaction molecule that interconnects IECs and IELs. Among the several monoclonal antibodies (mAbs) generated by immunizing porcine IECs, mAb (5-15-1) was shown to strongly react with IELs in addition to IECs. MALDI-TOF-MS and tandem MS analysis suggested that the antigen belongs to a family of human homophilic epithelial cell adhesion molecule (Ep-CAM). The amino acid sequence of porcine Ep-CAM showed 82.8%, 78.1%, and 76.8% homology compared to human, mouse, and rat Ep-CAM. Moreover, 5-15-1 specifically reacted with transfectant of porcine Ep-CAM. These data suggest that the Ep-CAM may act as a physical homophilic interaction molecule between IELs and IECs at the mucosal epithelium for providing immunological barrier as a first line of defense against mucosal infection.

Intestinal villous M cells: an antigen entry site in the mucosal epithelium

M cells located in the follicle-associated epithelium of Peyer's patches (PP) are shown to be the principal sites for the sampling of gut luminal antigens. Thus, PP have long been considered the gatekeepers of the mucosal immune system. Here, we report a distinct gateway for the uptake of gut bacteria: clusters of non-follicle-associated epithelium-associated Ulex europaeus agglutinin (UEA)-1(+) cells, which we have designated intestinal villous M cells. Interestingly, villous M cells are developed in various PP [or gut-associated lymphoid tissue (GALT)]-null mice, such as in utero lymphotoxin beta receptor (LTbetaR)-Ig-treated, lymphotoxin alpha (LTalpha)(-/-), tumor necrosis factor/LTalpha(-/-), and inhibition of differentiation 2 (Id2)(-/-) mice. Intestinal villous M cells have been observed to take up GFP-expressing Salmonella, Yersinia, and Escherichia coli-expressing invasin, as well as gut bacterial antigen for subsequent induction of antigen-specific immune responses. Thus, the identified villous M cells could be an alternative and PP-independent gateway for the induction of antigen-specific immune responses by means of the mucosal compartment.

Apoptosis by RGD-containing peptides observed in hemocytes of the Pacific oyster, Crassostrea gigas

We observed in vitro that after treatment with the Arg-Gly-Asp (RGD) peptide, non-spreading Crassostrea gigas hemocytes underwent cell death. Utilizing a combination of a Hoechst staining method and a DNA fragmentation assay, the typical features of apoptosis were shown, i.e. cell shrinkage, membrane blebbing, chromatin condensation, and DNA fragmentation. The hemocyte cell death caused by the RGD peptide appears to be sequence-specific, since no induction was shown in the alanine-substituted control peptide (RAD) treatment. Interestingly, the glutamic acid-substituted control peptide (RGE) also induced hemocytic cell death, but a different type of the death to that induced by the RGD peptide. This is the first report that specific peptides induce cell death in molluscan hemocytes.