It has recently been established that both acute human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections are accompanied by a dramatic and selective loss of memory CD4+ T cells predominantly from the mucosal surfaces. The mechanism underlying this depletion of memory CD4+ T cells (that is, T-helper cells specific to previously encountered pathogens) has not been defined. Using highly sensitive, quantitative polymerase chain reaction together with precise sorting of different subsets of CD4+ T cells in various tissues, we show that this loss is explained by a massive infection of memory CD4+ T cells by the virus. Specifically, 30-60% of CD4+ memory T cells throughout the body are infected by SIV at the peak of infection, and most of these infected cells disappear within four days. Furthermore, our data demonstrate that the depletion of memory CD4+ T cells occurs to a similar extent in all tissues. As a consequence, over one-half of all memory CD4+ T cells in SIV-infected macaques are destroyed directly by viral infection during the acute phase-an insult that certainly heralds subsequent immunodeficiency. Our findings point to the importance of reducing the cell-associated viral load during acute infection through therapeutic or vaccination strategies.

The molecular machinery governing glutamatergic-GABAergic neuronal subtype specification is unclear. Here we describe a cerebellar mutant, cerebelless, which lacks the entire cerebellar cortex in adults. The primary defect of the mutant brains was a specific inhibition of GABAergic neuron production from the cerebellar ventricular zone (VZ), resulting in secondary and complete loss of external germinal layer, pontine, and olivary nuclei during development. We identified the responsible gene, Ptf1a, whose expression was lost in the cerebellar VZ but was maintained in the pancreas in cerebelless. Lineage tracing revealed that two types of neural precursors exist in the cerebellar VZ: Ptf1a-expressing and -nonexpressing precursors, which generate GABAergic and glutamatergic neurons, respectively. Introduction of Ptf1a into glutamatergic neuron precursors in the dorsal telencephalon generated GABAergic neurons with representative morphological and migratory features. Our results suggest that Ptf1a is involved in driving neural precursors to differentiate into GABAergic neurons in the cerebellum.

Neutralizing antibodies (NAbs) can confer immunity to primate lentiviruses by blocking infection in macaque models of AIDS1–4. However, earlier studies of anti-HIV 1 NAbs administered to infected individuals or humanized mice, reported poor control of virus replication and the rapid emergence of resistant variants 5–7. A new generation of anti-HIV 1 monoclonal antibodies (mAbs), possessing extraordinary potency and breadth of neutralizing activity, has recently been isolated from infected individuals 8. These NAbs target different regions of the HIV 1 envelope glycoprotein including the CD4 binding site (bs), glycans located in the V1/V2, V3, and V4 regions, and the membrane proximal external region of gp419–14. We have examined two of the new antibodies, directed to the CD4 bs and the V3 region (3BNC117 and 10-1074 respectively) for their ability to block infection and suppress viremia in macaques infected with the R5 tropic SHIVAD8 virus, which emulates many of the pathogenic and immunogenic properties of HIV 1 during infections of rhesus macaques15,16. Either antibody alone can potently block virus acquisition. When administered individually to recently infected monkeys, the 10-1074 antibody caused a rapid decline in virus loads to undetectable levels for 4 to 7 days, followed by virus rebound during which neutralization resistant variants became detectable. When administered together, a single treatment rapidly suppressed plasma viremia for 3 to 5 weeks in some long-term chronically SHIV infected animals with low CD4+ T cell levels. A second cycle of anti-HIV 1 mAb therapy, administered to two previously treated animals, successfully controlled virus rebound. These results suggest that immunotherapy or a combination of immunotherapy plus conventional antiretroviral drugs might be useful as a treatment for chronically HIV-1 infected individuals experiencing immune dysfunction.

Highly potent and broadly neutralizing anti-HIV-1 antibodies (bNAbs) have been used to prevent and treat lentivirus infections in humanized mice, macaques and humans1–12. To determine whether the administration of combination bNAbs during the acute SHIV infection of rhesus macaques might lead to long-term control of virus replication, animals challenged with SHIVAD8-EO by mucosal or intravenous routes received a single 2-week course of 2 potent passively transferred bNAbs (3BNC117 and 10-107413,14). Viremia remained undetectable for 56–177 days, depending on bNAb half-life in vivo. Moreover, in the 13 treated monkeys, plasma virus loads subsequently declined to undetectable levels in 6 controller macaques. 4 additional animals maintained their CD4+ T cell counts and very low levels of viremia persisted for over 2 years. The frequency of cells carrying replication-competent virus was less than 1 per 106 circulating CD4+ T cells in the 6 controller macaques. Infusion of a T cell depleting anti-CD8β mAb to the controller animals led to a specific decline in levels of CD8+ T cells and rapid reappearance of plasma viremia. In contrast, macaques treated for 15 weeks with combination anti-retroviral therapy (cART), beginning on day 3 after infection, experienced sustained rebound plasma viremia when treatment was interrupted. We conclude that passive immunotherapy during the acute SHIV infection differs from cART in that it facilitates the emergence of potent CD8+ T cell immunity able to durably suppress virus replication.

Despite the success of potent anti-retroviral drugs in controlling HIV-1 infection, little progress has been made in generating an effective HIV-1 vaccine. Although passive transfer of anti-HIV-1 bNAbs can protect mice or macaques against a single high dose challenge with HIV or SIV/HIV chimeric viruses respectively1-8, the long-term efficacy of a passive antibody transfer approach for HIV-1 has not been examined. Based on the relatively long term protection conferred by Hepatitis A immune globulin, we tested the efficacy of a single injection (20mg/kg) of four anti-HIV-1 neutralizing monoclonal antibodies (MAbs) (VRC01, VRC01-LS, 3BNC117, and 10-10749-12) in blocking repeated weekly low dose virus challenges of the clade B SHIVAD8. Compared to control animals, which required 2 to 6 challenges (median=3 weeks) for infection, a single bNAb infusion prevented virus acquisition for up to 23 weeks. This effect depended on antibody potency and half-life. The highest levels of plasma neutralizing activity and correspondingly, the longest protection, were found in monkeys administered the more potent antibodies, 3BNC117 and 10-1074 (median=13 and 12.5 weeks respectively). VRC01, which showed lower plasma-neutralizing activity, protected for a shorter time (median=8 weeks). The introduction of a mutation that extends antibody half-life into the Fc domain of VRC01 increased median protection from 8 to 14.5 weeks. If administered in to populations at high risk for HIV-1 transmission, such an immunoprophylaxis regimen could have a major impact on virus transmission.

Five potent and broadly anti-HIV neutralizing monoclonal antibodies are able to block infection by two different SHIVs in monkeys. The authors show that antibodies targeting the outer glycan coat were the most effective and determined that titers of roughly 1:100 protected half the animals.

Serum characterization and antibody isolation are transforming our understanding of the humoral immune response to viral infection. Here, we show that epitope specificities of HIV-1-neutralizing antibodies in serum can be elucidated from the serum pattern of neutralization against a diverse panel of HIV-1 isolates. We determined "neutralization fingerprints" for 30 neutralizing antibodies on a panel of 34 diverse HIV-1 strains and showed that similarity in neutralization fingerprint correlated with similarity in epitope. We used these fingerprints to delineate specificities of polyclonal sera from 24 HIV-1-infected donors and a chimeric siman-human immunodeficiency virus-infected macaque. Delineated specificities matched published specificities and were further confirmed by antibody isolation for two sera. Patterns of virus-isolate neutralization can thus afford a detailed epitope-specific understanding of neutralizing-antibody responses to viral infection.

Broadly neutralizing antibodies (bNAbs) protect against HIV-1 infection, yet how they are generated during chronic infection remains unclear. It is known that T follicular helper (TFH) cells are needed to promote affinity maturation of B cells during an immune response; however, the role of TFH during HIV-1 infection is undefined within lymph node germinal centers (GCs). We use nonhuman primates to investigate the relationship in the early stage of chronic SHIVAD8 (simian-human immunodeficiency virus AD8) infection between envelope (Env)-specific TFH cells, Env-specific B cells, virus, and the generation of bNAbs during later infection. We found that both the frequency and quality of Env-specific TFH cells were associated with an expansion of Env-specific immunoglobulin G-positive GC B cells and broader neutralization across HIV clades. We also found a correlation between breadth of neutralization and the degree of somatic hypermutation in Env-specific memory B cells. Finally, we observed high viral loads and greater diversity of Env sequences in rhesus macaques that developed cross-reactive neutralization as compared to those that did not. These studies highlight the importance of boosting high-quality TFH populations as part of a robust vaccine regimen aimed at eliciting bNabs.