GALT CD4+PD-1hi T follicular helper (Tfh) cells repopulate after anti-retroviral therapy.

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections are characterized by dramatic alterations in the mucosal CD4 T cell compartment. Though viremia is effectively suppressed, and peripheral CD4 T cell numbers recover to near healthy levels after highly active anti-retroviral therapy (HAART), some of the dysfunctional consequences of HIV infection continue to persist during therapy. We hypothesized that CD4 T follicular helper (Tfh) cell deficiencies may play a role in this process. Using the macaque model we show that SIV infection was associated with a significant loss of Tfh cells in the GALT that drain the mesentery lining the gastrointestinal tract (GIT). Loss of Tfh cells significantly correlated with the depletion of the overall memory CD4 T cell compartment; most Tfh cells in the GALT expressed a CD95+CD28+ memory phenotype suggesting that infection of the memory compartment likely drives the loss of GALT Tfh cells during infection. Continuous anti-retroviral therapy (cART) was accompanied by a significant repopulation of Tfh cells in the GALT to levels similar to those of uninfected animals. Repopulating Tfh cells displayed significantly higher capacity to produce IL-21 as compared to SIV infected animals suggesting that cART fully restores Tfh cells that are functionally capable of supporting GC reactions in the GALT.

Chronic simian immunodeficiency virus infection is associated with contrasting phenotypes of dysfunctional Bcl6+ germinal center B cells or Bcl6- Bcl2+ non-germinal center B cells.

Human immunodeficiency virus (HIV) infection is characterized by dysfunctional B cell responses. Here we show that chronic simian immunodeficiency virus (SIV) infection is characterized by an expansion of either lymph node germinal center (GC) B cells that co-express Bcl6, Ki-67 and IL-21R and correlate with expanded T follicular helper (Tfh) cells or B cells that lack Bcl6, Ki-67 and IL-21R but express high levels of anti-apoptotic Bcl2 that negatively correlate with Tfh cells. The lack of Tfh cells likely contributes to persistence of dysfunctional non-proliferating B cells during chronic infection. These findings have implications for protective immunity in HIV-infected individuals who harbour low frequencies of Tfh cells.

Captopril mitigates splenomegaly and myelofibrosis in the Gata1low murine model of myelofibrosis.

Allogeneic stem cell transplantation is currently the only curative therapy for primary myelofibrosis (MF), while the JAK2 inhibitor, ruxolitinib. Has been approved only for palliation. Other therapies are desperately needed to reverse life-threatening MF. However, the cell(s) and cytokine(s) that promote MF remain unclear. Several reports have demonstrated that captopril, an inhibitor of angiotensin-converting enzyme that blocks the production of angiotensin II (Ang II), mitigates fibrosis in heart, lung, skin and kidney. Here, we show that captopril can mitigate the development of MF in the Gata1low mouse model of primary MF. Gata1low mice were treated with 79 mg/kg/d captopril in the drinking water from 10 to 12 months of age. At 13 months of age, bone marrows were examined for fibrosis, megakaryocytosis and collagen expression; spleens were examined for megakaryocytosis, splenomegaly and collagen expression. Treatment of Gata1low mice with captopril in the drinking water was associated with normalization of the bone marrow cellularity; reduced reticulin fibres, splenomegaly and megakaryocytosis; and decreased collagen expression. Our findings suggest that treating with the ACE inhibitors captopril has a significant benefit in overcoming pathological changes associated with MF.

Suppression of transforming growth factor ß receptor 2 and Smad5 is associated with high levels of microRNA miR-155 in the oral mucosa during chronic simian immunodeficiency virus infection.

Chronic human immunodeficiency virus and simian immunodeficiency virus (HIV and SIV) infections are characterized by mucosal inflammation in the presence of anti-inflammatory cytokines such as transforming growth factor ß (TGFß). The mechanisms for refractiveness to TGFß are not clear. Here we show that the expression of microRNA miR-155 was significantly upregulated in the oropharyngeal mucosa during chronic SIV infection and was coincident with downregulation of TGFß receptor 2 (TGFß-R2) and SMAD5, key TGFß signaling genes that harbor putative target sites for miR-155. Ectopic expression of miR-155 in vitro was found to significantly downregulate TGFß-R2 and Smad5 expression, suggesting a role for miR-155 in the suppression of TGFß-R2 and SMAD5 genes in vivo. The downregulation of TGFß signaling genes by miR-155 likely contributes to the nonresponsiveness to TGFß during SIV infection and may inadvertently aid in increased immune activation during HIV and SIV infections.

Early treatment with reverse transcriptase inhibitors significantly suppresses peak plasma IFNα in vivo during acute simian immunodeficiency virus infection.

Innate interferons (IFN) are comprised of multiple Type I and III subtypes. The in vivo kinetics of subtype responses during human immunodeficiency virus (HIV) infection is not well defined. Using the acute simian immunodeficiency virus (SIV) infection model, we show that plasma IFNα levels peak at day 10 post-infection (pi) after which they rapidly declined. The mRNA expression of Type I and III IFN subtypes were significantly elevated in the lymph nodes (LN) at day 10 pi. Though the expression levels of all subtypes declined by day 14-31 pi, numerous subtypes remained elevated suggesting that ongoing viral replication in LN continues to drive induction of these subtypes. Interestingly, treatment with reverse transcriptase (RT) inhibitors at day 7 pi significantly suppressed plasma IFNα responses by day 10 pi that significantly correlated with cell-associated SIV DNA loads suggesting that RT byproducts such as viral DNA likely plays a role in driving IFN responses during acute SIV infection. Quantification of Type I and III subtype transcripts in sorted subsets of LN CD4+ and CD8+ T cells, CD14+/CD14- monocytes/macrophages, and total CD11c/CD123+ dendritic cells (DC) at day 10 pi showed that DC expressed ∼3-4 log more subtype transcripts as compared to the other subsets. Taken together, our results provide new insights into the kinetics of innate interferon responses during early stages of infection, and provide evidence that DC's are a major in vivo source of innate IFN during acute SIV infection.

Suppressed Th17 levels correlate with elevated PIAS3, SHP2, and SOCS3 expression in CD4 T cells during acute simian immunodeficiency virus infection.

T helper 17 (Th17) cells play an important role in mucosal immune homeostasis and maintaining the integrity of the mucosal epithelial barrier. Loss of Th17 cells has been extensively documented during human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections. The lack of effective repopulation of Th17 cells has been associated with chronic immune activation mediated by the translocation of microbial products. Using ex vivo analysis of purified peripheral blood CD4 T cells from SIV-infected rhesus macaques, we show that the suppression of interleukin-17 (IL-17) expression correlated with upregulated expression of negative regulatory genes PIAS3, SHP2, and SOCS3 in CD4 T cells. Suppressed Th17 expression was accompanied by elevated levels of soluble CD14 (sCD14) and lipopolysaccharide binding protein (LBP) in the plasma during early stages of infection. Plasma viral loads rather than sCD14 or LBP levels correlated with acute immune activation. Additionally, we observed a significant increase in the expression of CD14 on peripheral blood monocytes that correlated with IL-23 expression and markers of microbial translocation. Taken together, our results provide new insights into the early events associated with acute SIV pathogenesis and suggest additional mechanisms playing a role in suppression of Th17 cells.

Zika Virus-A Reemerging Neurotropic Arbovirus Associated with Adverse Pregnancy Outcomes and Neuropathogenesis.

Zika virus (ZIKV) is a reemerging flavivirus that is primarily spread through bites from infected mosquitos. It was first discovered in 1947 in sentinel monkeys in Uganda and has since been the cause of several outbreaks, primarily in tropical and subtropical areas. Unlike earlier outbreaks, the 2015-2016 epidemic in Brazil was characterized by the emergence of neurovirulent strains of ZIKV strains that could be sexually and perinatally transmitted, leading to the Congenital Zika Syndrome (CZS) in newborns, and Guillain-Barre Syndrome (GBS) along with encephalitis and meningitis in adults. The immune response elicited by ZIKV infection is highly effective and characterized by the induction of both ZIKV-specific neutralizing antibodies and robust effector CD8+ T cell responses. However, the structural similarities between ZIKV and Dengue virus (DENV) lead to the induction of cross-reactive immune responses that could potentially enhance subsequent DENV infection, which imposes a constraint on the development of a highly efficacious ZIKV vaccine. The isolation and characterization of antibodies capable of cross-neutralizing both ZIKV and DENV along with cross-reactive CD8+ T cell responses suggest that vaccine immunogens can be designed to overcome these constraints. Here we review the structural characteristics of ZIKV along with the evidence of neuropathogenesis associated with ZIKV infection and the complex nature of the immune response that is elicited by ZIKV infection.

Sensitive and Accurate Quantification of Enterovirus-D68 (EV-D68) Viral Loads Using Droplet Digital PCR (ddPCR).

Enterovirus-D68 (EV-D68) is a reemerging virus that has been associated with numerous outbreaks in children in the past 10 years. Most assays examining viral infection kinetics have relied on the use of quantitative RT-PCR (qRT-PCR) assays as an assay of choice. Though valuable, there are inherent limitations that introduce variability, thereby reducing its value when comparing results across the field. Unlike the qRT-PCR assay that uses a standard curve to determine the copy number of viral RNA, the droplet digital PCR assay (ddPCR) directly quantifies the absolute number of copies within a given sample, which in turn makes the assay highly sensitive and accurate. Here, we have developed an EV-D68-specific ddPCR assay that effectively quantifies EV-D68 RNA copies in both cells and supernatants within a dynamic range of 6.7 × 10-3 copies/µL to 1.2 × 104 copies/µL of the sample. The assay was highly specific for a broad range of EV-D68 isolates (Fermon, US/MO/14-18947, US/MO/14-18949, US/KY/14-18953, USA/2018-23088, USA/2020-23336 and EV-D68-infected human nasal turbinate samples from the 2022 outbreak) without cross-reactivity to other viruses such as Enterovirus-A71 (EV-A71), Human Parechovirus (HPeV)-1 and -2, Coxsackievirus (CV)-B1, Human Coronavirus (HCoV)-NL63, SARS-CoV-2, Influenza-A and B, Rhinovirus, and Respiratory Syncytial Virus (RSV)-A2, which are known to cause infection in children. The assay was able to readily quantify EV-D68 in infected cells and supernatants along with nasal turbinate samples collected from children during the 2022 outbreak. Our results suggest that the assay can be readily translated to accurately quantify viral loads in tissues and body fluids such as plasma and lung or nasal aspirates.

Enterovirus-D68 - A Reemerging Non-Polio Enterovirus that Causes Severe Respiratory and Neurological Disease in Children.

The past decade has seen the global reemergence and rapid spread of enterovirus D68 (EV-D68), a respiratory pathogen that causes severe respiratory illness and paralysis in children. EV-D68 was first isolated in 1962 from children with pneumonia. Sporadic cases and small outbreaks have been reported since then with a major respiratory disease outbreak in 2014 associated with an increased number of children diagnosed with polio-like paralysis. From 2014-2018, major outbreaks have been reported every other year in a biennial pattern with > 90% of the cases occurring in children under the age of 16. With the outbreak of SARS-CoV-2 and the subsequent COVID-19 pandemic, there was a significant decrease in the prevalence EV-D68 cases along with other respiratory diseases. However, since the relaxation of pandemic social distancing protocols and masking mandates the number of EV-D68 cases have begun to rise again - culminating in another outbreak in 2022. Here we review the virology, pathogenesis, and the immune response to EV-D68, and discuss the epidemiology of EV-D68 infections and the divergence of contemporary strains from historical strains. Finally, we highlight some of the key challenges in the field that remain to be addressed.

Mucosal and peripheral Lin- HLA-DR+ CD11c/123- CD13+ CD14- mononuclear cells are preferentially infected during acute simian immunodeficiency virus infection.

Massive infection of memory CD4 T cells is a hallmark of early simian immunodeficiency virus (SIV) infection, with viral infection peaking at day 10 postinfection (p.i.), when a majority of memory CD4 T cells in mucosal and peripheral tissues are infected. It is not clear if mononuclear cells from the monocyte and macrophage lineages are similarly infected during this early phase of explosive HIV and SIV infections. Here we show that, at day 10 p.i., Lin(-) HLA-DR(+) CD11c/123(-) CD13(+) CD14(-) macrophages in the jejunal mucosa were infected, albeit at lower levels than CD4 memory T cells. Interestingly, Lin(-) HLA-DR(+) CD11c/123(-) CD13(+) CD14(-) macrophages in peripheral blood, like their mucosal counterparts, were preferentially infected compared to Lin(-) HLA-DR(+) CD11c/123(-) CD13(+) CD14(+) monocytes, suggesting that differentiated macrophages were selectively infected by SIV. CD13(+) CD14(-) macrophages expressed low levels of CD4 compared to CD4 T cells but expressed similar levels of CCR5 as lymphocytes. Interestingly, CD13(+) CD14(-) macrophages expressed Apobec3G at lower levels than CD13(+) CD14(+) monocytes, suggesting that intracellular restriction may contribute to the differential infection of mononuclear subsets. Taken together, our results suggest that CD13(+) CD14(-) macrophages in mucosal and peripheral tissues are preferentially infected very early during the course of SIV infection.

Significant mobilization of both conventional and regulatory T cells with AMD3100.

In this study, we used the rhesus macaque model to determine the impact that AMD3100 has on lymphocyte mobilization, both alone and in combination with G-CSF. Our results indicate that, unlike G-CSF, AMD3100 substantially mobilizes both B and T lymphocytes into the peripheral blood. This led to significant increases in the peripheral blood content of both effector and regulatory T-cell populations, which translated into greater accumulation of these cells in the resulting leukapheresis products. Notably, CD4(+)/CD25(high)/CD127(low)/FoxP3(+) Tregs were efficiently mobilized with AMD3100-containing regimens, with as much as a 4.0-fold enrichment in the leukapheresis product compared with G-CSF alone. CD8(+) T cells were mobilized to a greater extent than CD4(+) T cells, with accumulation of 3.7 ± 0.4-fold more total CD8+ T cells and 6.2 ± 0.4-fold more CD8(+) effector memory T cells in the leukapheresis product compared with G-CSF alone. Given that effector memory T-cell subpopulations may mediate less GVHD compared with other effector T-cell populations and that Tregs are protective against GVHD, our results indicate that AMD3100 may mobilize a GVHD-protective T-cell repertoire, which would be of benefit in allogeneic hematopoietic stem cell transplantation.

Uveitis-associated epitopes of retinal antigens are pathogenic in the humanized mouse model of uveitis and identify autoaggressive T cells.

Noninfectious uveitis is a leading cause of blindness and thought to involve autoimmune T cell responses to retinal proteins (e.g., retinal arrestin [soluble-Ag (S-Ag)]). There are no known biomarkers for the disease. Susceptibility is associated with HLA, but little is known about susceptible class II alleles or the potentially pathogenic epitopes that they present. Using a humanized HLA-transgenic mouse model of S-Ag-induced autoimmune uveitis, we identified several susceptible and resistant alleles of HLA-DR and -DQ genes and defined pathogenic epitopes of S-Ag presented by the susceptible alleles. The sequences of these epitopes overlap with some previously identified peptides of S-Ag ("M" and "N"), known to elicit memory responses in lymphocytes of uveitis patients. HLA-DR-restricted, S-Ag-specific CD4(+) T cells could be detected in blood and draining lymph nodes of uveitic mice with HLA class II tetramers and transferred the disease to healthy mice. Importantly, tetramer-positive cells were detected in peripheral blood of a uveitis patient. To our knowledge, these findings provide the first tangible evidence that an autoimmune response to retina is causally involved in pathogenesis of human uveitis, demonstrating the feasibility of identifying and isolating retinal Ag-specific T cells from uveitis patients and may facilitate their development as biomarkers for the disease.

Loss and dysregulation of Th17 cells during HIV infection.

Bacterial translocation across the damaged mucosal epithelium has emerged as a major paradigm for chronic immune activation observed during HIV infection. T helper 17 (Th17) cells are a unique lineage of T helper cells that are enriched in mucosal tissues and are thought to play a central role in protecting the integrity of the mucosal barrier and maintaining immune homeostasis at mucosal sites. Th17 cells are lost very early during the course of HIV infection, and their loss has been shown to correlate with bacterial translocation. Interestingly, Th17 cells are unable to completely recover from the early destruction even after successful antiretroviral therapy (ART). Here, we review some of the potential mechanisms for the loss and dysregulation of Th17 cells during HIV infection.

Genetic immunization in the lung induces potent local and systemic immune responses.

Successful vaccination against respiratory infections requires elicitation of high levels of potent and durable humoral and cellular responses in the lower airways. To accomplish this goal, we used a fine aerosol that targets the entire lung surface through normal respiration to deliver replication-incompetent recombinant adenoviral vectors expressing gene products from several infectious pathogens. We show that this regimen induced remarkably high and stable lung T-cell responses in nonhuman primates and that it also generated systemic and respiratory tract humoral responses of both IgA and IgG isotypes. Moreover, strong immunogenicity was achieved even in animals with preexisting antiadenoviral immunity, overcoming a critical hurdle to the use of these vectors in humans, who commonly are immune to adenoviruses. The immunogenicity profile elicited with this regimen, which is distinct from either intramuscular or intranasal delivery, has highly desirable properties for protection against respiratory pathogens. We show that it can be used repeatedly to generate mucosal humoral, CD4, and CD8 T-cell responses and as such may be applicable to other mucosally transmitted pathogens such as HIV. Indeed, in a lethal challenge model, we show that aerosolized recombinant adenoviral immunization completely protects ferrets against H5N1 highly pathogenic avian influenza virus. Thus, genetic immunization in the lung offers a powerful platform approach to generating protective immune responses against respiratory pathogens.

Direct intranodal tonsil vaccination with modified vaccinia Ankara vaccine protects macaques from highly pathogenic SIVmac251.

Human immunodeficiency virus (HIV) is a mucosally transmitted virus that causes immunodeficiency and AIDS. Developing efficacious vaccines to prevent infection is essential to control the epidemic. Protecting the vaginal and rectal mucosa, the primary routes of HIV entry has been a challenge given the significant compartmentalization between the mucosal and peripheral immune systems. We hypothesized that direct intranodal vaccination of mucosa associated lymphoid tissue (MALT) such as the readily accessible palatine tonsils could overcome this compartmentalization. Here we show that rhesus macaques primed with plasmid DNA encoding SIVmac251-env and gag genes followed by an intranodal tonsil MALT boost with MVA encoding the same genes protects from a repeated low dose intrarectal challenge with highly pathogenic SIVmac251; 43% (3/7) of vaccinated macaques remained uninfected after 9 challenges as compared to the unvaccinated control (0/6) animals. One vaccinated animal remained free of infection even after 22 challenges. Vaccination was associated with a ~2 log decrease in acute viremia that inversely correlated with anamnestic immune responses. Our results suggest that a combination of systemic and intranodal tonsil MALT vaccination could induce robust adaptive and innate immune responses leading to protection from mucosal infection with highly pathogenic HIV and rapidly control viral breakthroughs.

Vaccination preserves CD4 memory T cells during acute simian immunodeficiency virus challenge.

Acute simian immunodeficiency virus (SIV)/human immunodeficiency virus infection is accompanied by a massive destruction of CD4 memory T cells across all the tissue compartments. These early events set the course toward disease progression and immunodeficiency. Here, we demonstrate that prior vaccination reduces this destruction during acute SIV Mac251 infection, leading to better survival and long-term outcome. Systemic vaccination with a DNA-prime recombinant adenovirus boost regimen preserved memory CD4 T cells throughout the body. The vaccine regimen induced broad CD4 and CD8 T cell responses in all tissues examined and, importantly, induced antibodies that neutralized the primary isolate of SIV used for challenge. Finally, we demonstrate that the extent of preservation of the CD4 memory compartment during the acute phase provides a strong predictor for subsequent progression to death. Our data provide a mechanism to explain clinical observations that acute-phase viral loads predict long-term disease progression and underscore the need for interventions that protect against early destruction of CD4 memory T cells during acute infection.

Toll-like receptor agonists influence the magnitude and quality of memory T cell responses after prime-boost immunization in nonhuman primates.

There is a remarkable heterogeneity in the functional profile (quality) of T cell responses. Importantly, the magnitude and/or quality of a response required for protection may be different depending on the infection. Here, we assessed the capacity of different Toll like receptor (TLR)-binding compounds to influence T helper cell (Th)1 and CD8+ T cell responses when used as adjuvants in nonhuman primates (NHP) with HIV Gag as a model antigen. NHP were immunized with HIV Gag protein emulsified in Montanide ISA 51, an oil-based adjuvant, with or without a TLR7/8 agonist, a TLR8 agonist, or the TLR9 ligand cytosine phosphate guanosine oligodeoxynucleotides (CpG ODN), and boosted 12 wk later with a replication-defective adenovirus-expressing HIV-Gag (rAD-Gag). Animals vaccinated with HIV Gag protein/Montanide and CpG ODN or the TLR7/8 agonist had higher frequencies of Th1 responses after primary immunization compared to all other vaccine groups. Although the rAD-Gag boost did not elevate the frequency of Th1 memory cytokine responses, there was a striking increase in HIV Gag-specific CD8+ T cell responses after the boost in all animals that had received a primary immunization with any of the TLR adjuvants. Importantly, the presence and type of TLR adjuvant used during primary immunization conferred stability and dramatically influenced the magnitude and quality of the Th1 and CD8+ T cell responses after the rAD-Gag boost. These data provide insights for designing prime-boost immunization regimens to optimize Th1 and CD8+ T cell responses.

High frequencies of resting CD4+ T cells containing integrated viral DNA are found in rhesus macaques during acute lentivirus infections.

We and others have reported that the vast majority of virus-producing CD4(+) T cells during the acute infection of rhesus macaques with simian immunodeficiency virus (SIV) or CXCR4 (X4)-using simian/human immunodeficiency viruses (SHIVs) exhibited a nonactivated phenotype. These findings have been extended to show that resting CD4(+) T lymphocytes collected from SIV- or X4-SHIV-infected animals during the first 10 days of infection continue to release virus ex vivo. Furthermore, we observed high frequencies of integrated viral DNA (up to 5.1 x 10(4) DNA copies per 10(5) cells) in circulating resting CD4(+) T cells during the first 10 days of the infection. Integration of SIV DNA was detected only in memory CD4(+) T cells and SHIVs preferentially integrated into resting naïve CD4(+) T cells. Taken together, these results show that during the acute infection large numbers of resting CD4(+) T cells carry integrated nonhuman primate lentiviral DNA and are the major source of progeny virions irrespective of coreceptor usage. Prompt and sustained interventions are therefore required to block the rapid systemic dissemination of virus and prevent an otherwise fatal clinical outcome.

Immune system development during early childhood in tropical Latin America: evidence for the age-dependent down regulation of the innate immune response.

The immune response that develops in early childhood underlies the development of inflammatory diseases such as asthma and there are few data from tropical Latin America (LA). This study investigated the effects of age on the development of immunity during the first 5 years of life by comparing innate and adaptive immune responses in Ecuadorian children aged 6-9 months, 22-26 months, and 48-60 months. Percentages of naïve CD4+ T cells declined with age while those of memory CD4(+) and CD8(+) T cells increased indicating active development of the immune system throughout the first five years. Young infants had greater innate immune responses to TLR agonists compared to older children while regulatory responses including SEB-induced IL-10 and percentages of FoxP3(+) T-regulatory cells decreased with age. Enhanced innate immunity in early life may be important for host defense against pathogens but may increase the risk of immunopathology.

CD8+ cell depletion of SHIV89.6P-infected macaques induces CD4+ T cell proliferation that contributes to increased viral loads.

Previous studies have shown that depletion of CD8(+) cells during acute and chronic simian immunodeficiency virus (SIV) infection leads to increased viral replication, morbidity, and mortality, which have been attributed to loss of CD8(+) T cell-mediated control of SIV. However, these studies did not exclude that CD8(+) cell depletion increased homeostatic proliferation of CD4(+) T cells, resulting in increased viral targets and, therefore, viral rebound. Chronically SHIV89.6P-infected cynomolgus macaques were CD8(+) cell-depleted, and the frequency, cell number, and phenotype of CD4(+) T cells and viral infection were examined using flow cytometry and quantitative real-time PCR. The frequency and number of Ki-67-expressing CD4(+) T cells were increased with CD8(+) cell depletion. This proliferation of CD4(+) T cells occurred even in animals with no rebound of viral loads. Most of the proliferating cells were effector memory CD4(+) T cells. Plasma simian HIV (SHIV) RNA copies positively correlated with proliferating CD4(+) T cells and SHIV DNA copies in Ki-67(+) CD4(+) T cells. Although this study does not exclude an important role for virus-specific CD8(+) T cells in SIV and SHIV infection, our data suggest that homeostatic proliferation is an important contributor to increases in plasma viremia that follow CD8(+) cell depletion.