The dysfunction of T follicular helper cells

Dysregulation of circulating T follicular helper cell subsets and their potential role in the pathogenesis of syphilis

Introduction

The role of the host immune response could be critical in the development of Treponema pallidum (Tp) infection in individuals with latent syphilis. This study aims to investigate the alterations in T follicular helper T (Tfh) cell balance among patients with secondary syphilis and latent syphilis.

Methods

30 healthy controls (HCs), 24 secondary syphilis patients and 41 latent syphilis patients were enrolled. The percentages of total Tfh, ICOS+ Tfh, PD-1+ Tfh, resting Tfh, effector Tfh, naïve Tfh, effector memory Tfh, central memory Tfh,Tfh1, Tfh2, and Tfh17 cells in the peripheral blood were all determined by flow cytometry.

Results

The percentage of total Tfh cells was significantly higher in secondary syphilis patients compared to HCs across various subsets, including ICOS+ Tfh, PD-1+ Tfh, resting Tfh, effector Tfh, naïve Tfh, effector memory Tfh, central memory Tfh, Tfh1, Tfh2, and Tfh17 cells. However, only the percentages of ICOS+ Tfh and effector memory Tfh cells showed significant increases in secondary syphilis patients and decreases in latent syphilis patients. Furthermore, the PD-1+ Tfh cells, central memory Tfh cells, and Tfh2 cells showed significant increases in latent syphilis patients, whereas naïve Tfh cells and Tfh1 cells exhibited significant decreases in secondary syphilis patients when compared to the HCs. However, no significant change was found in resting Tfh and effector Tfh in HCs and secondary syphilis patients or latent syphilis patients.

Discussion

Dysregulated ICOS+ Tfh or effector memory Tfh cells may play an important role in immune evasion in latent syphilis patients.

Immune Responses and Viral Persistence in Simian/Human Immunodeficiency Virus SHIV.C.CH848-Infected Rhesus Macaques

SHIVs have been extensively used in a nonhuman primate (NHP) model for HIV research. In this study, we investigated viral reservoirs in tissues and immune responses in an NHP model inoculated with newly generated transmitted/founder HIV-1 clade C-based SHIV.C.CH848.

Chimeric simian/human immunodeficiency viruses (SHIVs) are widely used in nonhuman primate models to recapitulate human immunodeficiency virus (HIV) infection in humans, yet most SHIVs fail to establish persistent viral infection. We investigated immunological and virological events in rhesus macaques infected with the newly developed SHIV.C.CH848 (SHIVC) and treated with combined antiretroviral therapy (cART). Similar to HIV/simian immunodeficiency virus (SIV) infection, SHIV.C.CH848 infection established viral reservoirs in CD4⁺ T cells and myeloid cells, accompanied by productive infection and depletion of CD4⁺ T cells in systemic and lymphoid tissues throughout SHIV infection. Despite 6 months of cART-suppressed viral replication, integrated proviral DNA levels remained stable, especially in CD4⁺ T cells, and the viral rebound was also observed after ART interruption. Autologous neutralizing antibodies to the parental HIV-1 strain CH848 were detected, with limited viral evolution at 5 months postinfection. In comparison, heterogenous neutralizing antibodies in SHIV.C.CH848-infected macaques were not detected except for 1 (1 of 10) animal at 2 years postinfection. These findings suggest that SHIV.C.CH848, a novel class of transmitted/founder SHIVs, can establish sustained viremia and viral reservoirs in rhesus macaques with clinical immunodeficiency consequences, providing a valuable SHIV model for HIV research.

IMPORTANCE SHIVs have been extensively used in a nonhuman primate (NHP) model for HIV research. In this study, we investigated viral reservoirs in tissues and immune responses in an NHP model inoculated with newly generated transmitted/founder HIV-1 clade C-based SHIV.C.CH848. The data show that transmitted founder (T/F) SHIVC infection of macaques more closely recapitulates the virological and clinical features of HIV infection, including persistent viremia and viral rebound once antiretroviral therapy is discontinued. These results suggest this CCR5-tropic, SHIVC strain is valuable for testing responses to HIV vaccines and therapeutics.

Widespread B cell perturbations in HIV-1 infection afflict naive and marginal zone B cells

Liechti et al. demonstrate severe B cell perturbations in HIV-1 infection beyond described effects on memory B cells. Naive and marginal zone B cells down-regulate CD21 and display chemokine receptor and activation marker signatures associated with inflammation and diminished response to stimulation.

Perturbations in B cells are a hallmark of HIV-1 infection. This is signified by increased numbers of exhausted CD21^neg memory B cells, driven by continuous antigen-specific and bystander activation. Using high-dimensional flow cytometry, we demonstrate that this exhausted phenotype is also prevalent among peripheral antigen-inexperienced naive and marginal zone (MZ) B cells in acute and chronic HIV-1 infection. A substantial fraction of naive and MZ B cells exhibit down-regulated CD21 levels and diminished response to B cell receptor (BCR)–dependent stimulation. Compared with CD21^pos subsets, the CD21^neg naive and MZ B cells differ in the expression of chemokine receptors and activation markers. Effective antiretroviral treatment normalizes peripheral naive and MZ B cell populations. Our results emphasize a more widely spread impairment of B cells in HIV-1 infection than previously appreciated, including antigen-inexperienced cells. This highlights the importance of monitoring functional capacities of naive B cells in HIV-1 infection, as exhausted CD21^neg naive B cells may severely impair induction of novel B cell responses.

Nonhuman Primate Models and Understanding the Pathogenesis of HIV Infection and AIDS

Research using nonhuman primates (NHPs) as models for human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS) has resulted in tremendous achievements not only in the prevention and treatment of HIV, but also in biomedical research more broadly. Once considered a death sentence, HIV infection is now fairly well controlled with combination antiretroviral treatments, almost all of which were first tested for efficacy and safety in nonhuman primates or other laboratory animals. Research in NHP has led to "dogma changing" discoveries in immunology, infectious disease, and even our own genetics. We now know that many of our genes are retroviral remnants, or developed in response to archaic HIV-like retroviral infections. Early studies involving blood from HIV patients and in experiments in cultured tissues contributed to confusion regarding the cause of AIDS and impeded progress in the development of effective interventions. Research on the many retroviruses of different NHP species have broadened our understanding of human immunology and perhaps even our origins and evolution as a species. In combination with recent advances in molecular biology and computational analytics, research in NHPs has unique potential for discoveries that will directly lead to new cures for old human and animal diseases, including HIV/AIDS.

T Cell Subsets in the Germinal Center: Lessons from the Macaque Model

Germinal centers (GCs) are organized lymphoid tissue microstructures where B cells proliferate and differentiate into memory B cells and plasma cells. A few distinctive subsets of highly specialized T cells gain access to the GCs by expressing the B cell zone–homing C-X-C chemokine receptor type 5 (CXCR5) while losing the T cell zone–homing chemokine receptor CCR7. Help from T cells is critical to induce B cell proliferation and somatic hyper mutation and to limit GC reactions. CD4⁺ T follicular helper (T_FH) cells required for the formation of GCs and for the generation of long-lived, high-affinity B cells. Regulatory CD4⁺ (T_FR) and CD8⁺ T cells co-localize with T_FH cells and keep their expansion in check, thus limiting GC reactions. A cytotoxic CXCR5^pos CD8⁺ T cell subset has been described in GCs in humans: although low in number, GC CD8⁺ T cells can expand rapidly during certain viral infections. Because these subsets find their home in secondary lymphoid tissues (lymph nodes and spleen) that are difficult to obtain in humans, GC–homing T cells have been extensively studied in mice. Nevertheless, significant limitations in using this model, such as evolutionary divergences between mice and humans and the lack of an optimal mouse model for certain human diseases, have prompted investigators to characterize GC–homing T cells in macaques instead. This review will focus on discoveries made in macaques, particularly in the non-human primate models of simian immunodeficiency virus and simian–human immunodeficiency virus infection. Indeed, experimental studies in these models have allowed researchers to gain insight into the relative role of follicular T cell subsets in HIV progression, virus persistence, and specific B cell responses induced by HIV vaccines. These discoveries have prompted the testing of novel approaches aimed to manipulate follicular T cells to increase the efficacy of HIV vaccines and to eliminate HIV reservoirs.

Distribution of circulating T follicular helper cell subsets is altered in immunoglobulin A vasculitis in children

Background

Immunoglobulin A vasculitis (IgAV) is the most common vasculitis in children. Previously, we demonstrated that patients with IgAV show abnormal proliferation of cluster of differentiation (CD)4⁺C-X-C chemokine receptor type (CXCR)5⁺ follicular helper T (Tfh) cells. Here, we explored the status of Tfh cell subsets and plasma cytokine levels in patients with IgAV.

Methods

CD4⁺CXCR5⁺CD45RA⁻, CD45RA^-CXCR3⁺CCR6⁻, CD45RA⁻CXCR3⁻CCR6⁻, CD45RA⁻CXCR3⁻CCR6⁺, and CD45RA⁻CXCR3⁺CCR6⁺ Tfh cell fractions and plasma concentrations of interferon-γ, interleukin (IL)-4, and IL-17A were evaluated by flow cytometry and a flow cytometric bead array, respectively, in 30 patients with IgAV and 15 healthy controls (HCs).

Results

Tfh2 and Tfh17 cell fractions were larger and the Tfh2+Tfh17/Tfh1 ratio and plasmaIL-4 and -17A levels were higher in patients with IgAV than in the HCs. Only Tfh1 cell counts were reduced in the abdominal subtype. The elevations in Tfh2 and Tfh17 cell counts and plasma IL-4 levels were abrogated by treatment. Tfh2 cell number was positively correlated with serum IgA and plasma IL-4 levels, but negatively correlated with the serum C4 concentration, while Tfh17 cell number was positively correlated with the serum IgA level and Tfh2 cell counts.

Conclusions

Abnormally high numbers of Tfh2 and Tfh17 cells are linked to the occurrence and development of IgAV, but are not specific to the abdominal type. Only Tfh1 cells play a critical role in abdominal-type IgAV.

How Germinal Centers Evolve Broadly Neutralizing Antibodies: the Breadth of the Follicular Helper T Cell Response

Many HIV-1-infected patients evolve broadly neutralizing antibodies (bnAbs). This evolutionary process typically takes several years and is poorly understood as selection taking place in germinal centers occurs on the basis of antibody affinity. B cells with the highest-affinity receptors tend to acquire the most antigen from the follicular dendritic cell (FDC) network and present the highest density of cognate peptides to follicular helper T (Tfh) cells, which provide survival signals to the B cell. bnAbs are therefore expected to evolve only when the B cell lineage evolving breadth is consistently capturing and presenting more peptides to Tfh cells than other lineages of more specific B cells. Here we develop mathematical models of Tfh cells in germinal centers to explicitly define the mechanisms of selection in this complex evolutionary process. Our results suggest that broadly reactive B cells presenting a high density of peptides bound to major histocompatibility complex class II molecules (pMHC) are readily outcompeted by B cells responding to lineages of HIV-1 that transiently dominate the within host viral population. Conversely, if broadly reactive B cells acquire a large variety of several HIV-1 proteins from the FDC network and present a high diversity of several pMHC, they can be rescued by a large fraction of the Tfh cell repertoire in the germinal center. Under such circumstances the evolution of bnAbs is much more consistent. Increasing either the magnitude of the Tfh cell response or the breadth of the Tfh cell repertoire markedly facilitates the evolution of bnAbs. Because both the magnitude and breadth can be increased by vaccination with several HIV-1 proteins, this calls for experimental testing.IMPORTANCE Many HIV-infected patients slowly evolve antibodies that can neutralize a large variety of viruses. Such broadly neutralizing antibodies (bnAbs) could in the future become therapeutic agents. bnAbs appear very late, and patients are typically not protected by them. At the moment, we fail to understand why this takes so long and how the immune system selects for broadly neutralizing capacity. Typically, antibodies are selected based on affinity and not on breadth. We developed mathematical models to study two different mechanisms by which the immune system can select for broadly neutralizing capacity. One of these is based upon the repertoire of different follicular helper T (Tfh) cells in germinal centers. We suggest that broadly reactive B cells may interact with a larger fraction of this repertoire and demonstrate that this would select for bnAbs. Intriguingly, this suggests that broadening the Tfh cell repertoire by vaccination may speed up the evolution of bnAbs.

B-cell abnormalities and impact on antibody response in HIV infection

PURPOSE OF REVIEW

The purpose of the present review is to provide an update on the current development in the field of broadly neutralizing antibodies (bNabs) and their potential use in the prevention and therapeutic settings, and an evaluation of the B-cell abnormalities that may impair antibody responses in HIV infection.

RECENT FINDINGS

Major advances have been achieved in the characterization of bNabs directed against different vulnerable regions of HIV Envelope (Env). Recent observations have clearly demonstrated the ability of bNabs to prevent HIV infection in the nonhuman primate model of HIV infection and to suppress viremia in individuals with chronic HIV infection in the absence of antiretroviral therapy. Furthermore, substantial advances have also been obtained in the development of HIV Env proteins and immunization strategies inducing bNabs in small animal models. Several studies have also shed light on the B-cell abnormalities associated with the viremic phase of HIV infection that cause impaired B-cell maturation and antibody responses. Of note, preliminary observations have provided evidence for a correlation between the expansion of a specific population of B cells, for example, germinal center B cells, the expansion of T follicular helper cells (Tfh), and the generation of neutralizing antibodies.

SUMMARY

The recent observations on the antiviral effects of bNabs in vivo indicate that bNabs may play a central role in both the prevention and the therapeutic settings. The identification of the role of germinal center B cells and Tfh cells as critical components of the immune response leading to the generation of neutralizing antibodies, will allow the development of specific immunization strategies for the stimulation of germinal center B cells and Tfh cells. A lot of work still remains to be done for the delineation of B-cell and Tfh cell biology from human lymphoid tissues and in the development of HIV Env proteins and immunization strategies leading to the generation of bNabs.

Antiviral Functions of Human Immunodeficiency Virus Type 1 (HIV-1)-Specific IgG Antibodies: Effects of Antiretroviral Therapy and Implications for Therapeutic HIV-1 Vaccine Design

Contemporary antiretroviral therapy (ART) is effective and tolerable for long periods of time but cannot eradicate human immunodeficiency virus type 1 (HIV-1) infection by either elimination of viral reservoirs or enhancement of HIV-1-specific immune responses. Boosting "protective" HIV-1-specific immune responses by active or passive immunization will therefore be necessary to control or eradicate HIV-1 infection and is currently the topic of intense investigation. Recently reported studies conducted in HIV patients and non-human primate (NHP) models of HIV-1 infection suggest that HIV-1-specific IgG antibody responses may contribute to the control of HIV-1 infection. However, production of IgG antibodies with virus neutralizing activity by vaccination remains problematic and while vaccine-induced natural killer cell-activating IgG antibodies have been shown to prevent the acquisition of HIV-1 infection, they may not be sufficient to control or eradicate established HIV-1 infection. It is, therefore, important to consider other functional characteristics of IgG antibody responses. IgG antibodies to viruses also mediate opsonophagocytic antibody responses against virions and capsids that enhance the function of phagocytic cells playing critical roles in antiviral immune responses, particularly conventional dendritic cells and plasmacytoid dendritic cells. Emerging evidence suggests that these antibody functions might contribute to the control of HIV-1 infection. In addition, IgG antibodies contribute to the intracellular degradation of viruses via binding to the cytosolic fragment crystallizable (Fc) receptor tripartite motif containing-21 (TRIM21). The functional activity of an IgG antibody response is influenced by the IgG subclass content, which affects binding to antigens and to Fcγ receptors on phagocytic cells and to TRIM21. The IgG subclass content and avidity of IgG antibodies is determined by germinal center (GC) reactions in follicles of lymphoid tissue. As HIV-1 infects cells in GCs and induces GC dysfunction, which may persist during ART, strategies for boosting HIV-1-specific IgG antibody responses should include early commencement of ART and possibly the use of particular antiretroviral drugs to optimize drug levels in lymphoid follicles. Finally, enhancing particular functions of HIV-1-specific IgG antibody responses by using adjuvants or cytokines to modulate the IgG subclass content of the antibody response might be investigated in NHP models of HIV-1 infection and during trials of therapeutic vaccines in HIV patients.

Preservation of Peripheral T Follicular Helper Cell Function in HIV Controllers

The maturation process of high-affinity antibodies is a result of intricate interactions between B cells and follicular helper T (Tfh) cells occurring in lymphoid germinal centers. HIV infection induces significant chronic immune activation, phenotypic skewing, and inflammation driven by years of continuous viral replication. High levels of viremia as well as immune activation and dysfunction have been demonstrated to have a perturbing impact on the B cell memory compartment and contribute to B cell exhaustion. Counterintuitively, the factors associated with perturbation of the B cell compartment seem to be favorable for the generation of highly affinity-matured Env-specific antibodies in a minority of HIV-infected individuals. Thus, the impact of HIV antigenemia on B cells and Tfh cell interactions warrants further exploration. We therefore studied immunophenotypes of HIV-specific B cells in individuals with differing levels of viral control using HIV Env gp120 probes and characterized the functionality of matched T cells in peripheral blood. While CXCR5⁺ CD4⁺ T cells were significantly diminished in HIV progressors, we found that a small subset of gp120-specific interleukin-21 (IL-21)-secreting CXCR5⁺ CD4⁺ T cells were significantly associated with gp120-specific B cell frequencies. In contrast, neither bulk CXCR5⁺ CD4⁺ T cells nor other HIV antigen specificities were associated with gp120-specific B cell levels. HIV-specific B cells derived from elite controllers displayed greater amounts of gp120-specific B cells in the resting memory subset, whereas HIV-specific B cells in progressors accumulated in tissue-like and activated memory subsets. Furthermore, CXCR5⁺ CD4⁺ T cells from elite controllers showed a stronger ex vivo capacity to induce B cell maturation and immunoglobulin class switching than cells from HIV progressors.IMPORTANCE Dissecting the factors that are involved in B cell maturation and antibody development is important for HIV vaccine design. In this study, we found that HIV Env-specific CXCR5⁺ CD4⁺ T cells that secrete interleukin-21 are strongly associated with B cell memory phenotypes and function. Moreover, we found that the immune responses of HIV controllers showed intrinsically better helper activity than those of HIV progressors.

B-cell responses to HIV infection

The induction of neutralizing antibodies directed against the human immunodeficiency virus (HIV) has received considerable attention in recent years, in part driven by renewed interest and opportunities for antibody-based strategies for prevention such as passive transfer of antibodies and the development of preventive vaccines, as well as immune-based therapeutic interventions. Advances in the ability to screen, isolate, and characterize HIV-specific antibodies have led to the identification of a new generation of potent broadly neutralizing antibodies (bNAbs). The majority of these antibodies have been isolated from B cells of chronically HIV-infected individuals with detectable viremia. In this review, we provide insight into the phenotypic and functional attributes of human B cells, with a focus on HIV-specific memory B cells and plasmablasts/cells that are responsible for sustaining humoral immune responses against HIV. We discuss the abnormalities in B cells that occur in HIV infection both in the peripheral blood and lymphoid tissues, especially in the setting of persisting viremia. Finally, we consider the opportunities and drawbacks of intensively interrogating antibodies isolated from HIV-infected individuals to guide strategies aimed at developing effective antibody-based vaccine and therapeutic interventions for HIV.

Natural infection as a blueprint for rational HIV vaccine design

So far, the development of a human immunodeficiency virus (HIV) vaccine has been unsuccessful. However, recent progress in the field of broadly neutralizing antibodies (bNAbs) has reinvigorated the search for an HIV vaccine. bNAbs develop in a minority of HIV infected individuals and passive transfer of these bNAbs to non-human primates provides protection from HIV infection. Studies in a number of HIV infected individuals on bNAb maturation alongside viral evolution and escape have shed light on the features important for bNAb elicitation. Here we review the observations from these studies, and how they influence the rational design of HIV vaccines.

The challenges of modelling antibody repertoire dynamics in HIV infection

Antibody affinity maturation by somatic hypermutation of B-cell immunoglobulin variable region genes has been studied for decades in various model systems using well-defined antigens. While much is known about the molecular details of the process, our understanding of the selective forces that generate affinity maturation are less well developed, particularly in the case of a co-evolving pathogen such as HIV. Despite this gap in understanding, high-throughput antibody sequence data are increasingly being collected to investigate the evolutionary trajectories of antibody lineages in HIV-infected individuals. Here, we review what is known in controlled experimental systems about the mechanisms underlying antibody selection and compare this to the observed temporal patterns of antibody evolution in HIV infection. We describe how our current understanding of antibody selection mechanisms leaves questions about antibody dynamics in HIV infection unanswered. Without a mechanistic understanding of antibody selection in the context of a co-evolving viral population, modelling and analysis of antibody sequences in HIV-infected individuals will be limited in their interpretation and predictive ability.

Persistent Simian Immunodeficiency Virus Infection Drives Differentiation, Aberrant Accumulation, and Latent Infection of Germinal Center Follicular T Helper Cells

CD4(+) follicular T helper (Tfh) cells play a prominent role in humoral immune responses, but the mechanisms of their accumulation and infection in AIDS remain unclear. Here we found that germinal center (GC) Tfh cells, defined here as CXCR5(+) PD-1(HIGH) CD4(+) T cells, do not express the HIV coreceptor CCR5 yet serve as a latent reservoir in GCs. With disease progression, an expansion of GC Tfh cells is accompanied by increases in dysfunctional CD8(+) T cells. In contrast, Tfh precursor (CXCR5(-) CD4(+) T) cells in lymph nodes do express CCR5 and differentiate into GC Tfh cells following interleukin-6 (IL-6) and IL-21 stimulation, and viral DNA is detectable in fully differentiated GC Tfh cells ex vivo. This suggests that SIV-infected GC Tfh cells may be derived from Tfh precursor cell subsets that become infected in marginal zones and then migrate into GCs as fully mature GC Tfh cells that serve as persistent virus reservoirs. These findings suggest that viral persistence in lymph nodes drives compensatory differentiation, aberrant accumulation, and latent infection of GC Tfh cells, resulting in marked impairment of humoral immune responses.

IMPORTANCE

Generation of antibodies that can effectively eliminate viruses requires interactions of B cells with highly specialized T cells in GCs of lymphoid tissues called follicular T helper cells. Here we show that in simian immunodeficiency virus infection, these cells are initially infected in a precursor stage that leads to alterations in their homing, accumulation, and function that may be responsible for the inability of human immunodeficiency virus-infected patients to generate effective antibody responses.

Persistent Simian Immunodeficiency Virus Infection Causes Ultimate Depletion of Follicular Th Cells in AIDS

CD4(+) T follicular helper (Tfh) cells are critical for the generation of humoral immune responses to pathogenic infections, providing help for B cell development, survival, and affinity maturation of Abs. Although CD4(+) Tfh cells are reported to accumulate in HIV or SIV infection, we found that germinal center Tfh cells, defined in this study as CXCR5(+)PD-1(HIGH)CD4(+) T cells, did not consistently accumulate in chronically SIV-infected rhesus macaques compared with those infected with less pathogenic simian HIV, vaccinated and SIVmac-challenged, or SIVmac-infected Mamu-A*01(+) macaques, all of which are associated with some control of virus replication and slower disease progression. Interestingly, CXCR5(+)PD-1(HIGH) Tfh cells in lymphoid tissues were eventually depleted in macaques with AIDS compared with the other cohorts. Chronic activation and proliferation of CXCR5(+)PD-1(HIGH) Tfh were increased, but PD-L2 expression was downregulated on B cells, possibly resulting in germinal center Tfh cell apoptosis. Together, these findings suggest that changes in CXCR5(+)PD-1(HIGH) Tfh cells in lymph nodes correlate with immune control during infection, and their loss or dysregulation contribute to impairment of B cell responses and progression to AIDS.

Low SAMHD1 expression following T-cell activation and proliferation renders CD4+ T cells susceptible to HIV-1

Objectives:

HIV-1 replication depends on the state of cell activation and division. It is established that SAMHD1 restricts HIV-1 infection of resting CD4⁺ T cells. The modulation of SAMHD1 expression during T-cell activation and proliferation, however, remains unclear, as well as a role for SAMHD1 during HIV-1 pathogenesis.

Methods:

SAMHD1 expression was assessed in CD4⁺ T cells after their activation and in-vitro HIV-1 infection. We performed phenotype analyzes using flow cytometry on CD4⁺ T cells from peripheral blood and lymph nodes from cohorts of HIV-1-infected individuals under antiretroviral treatment or not, and controls.

Results:

We show that SAMHD1 expression decreased during CD4⁺ T-cell proliferation in association with an increased susceptibility to in-vitro HIV-1 infection. Additionally, circulating memory CD4⁺ T cells are enriched in cells with low levels of SAMHD1. These SAMHD1^low cells are highly differentiated, exhibit a large proportion of Ki67⁺ cycling cells and are enriched in T-helper 17 cells. Importantly, memory SAMHD1^low cells were depleted from peripheral blood of HIV-infected individuals. We also found that follicular helper T cells present in secondary lymphoid organs lacked the expression of SAMHD1, which was accompanied by a higher susceptibility to HIV-1 infection in vitro.

Conclusion:

We demonstrate that SAMHD1 expression is decreased during CD4⁺ T-cell activation and proliferation. Also, CD4⁺ T-cell subsets known to be more susceptible to HIV-1 infection, for example, T-helper 17 and follicular helper T cells, display lower levels of SAMHD1. These results pin point a role for SAMHD1 expression in HIV-1 infection and the concomitant depletion of CD4⁺ T cells.