Intestinal macrophages display reduced permissiveness to human immunodeficiency virus 1 and decreased surface CCR5

HIV-1 Myeloid Reservoirs - Contributors to Viral Persistence and Pathogenesis

PURPOSE OF REVIEW

HIV reservoirs are the main barrier to cure. CD4+ T cells have been extensively studied as the primary HIV-1 reservoir. However, there is substantial evidence that HIV-1-infected myeloid cells (monocytes/macrophages) also contribute to viral persistence and pathogenesis.

RECENT FINDINGS

Recent studies in animal models and people with HIV-1 demonstrate that myeloid cells are cellular reservoirs of HIV-1. HIV-1 genomes and viral RNA have been reported in circulating monocytes and tissue-resident macrophages from the brain, urethra, gut, liver, and spleen. Importantly, viral outgrowth assays have quantified persistent infectious virus from monocyte-derived macrophages and tissue-resident macrophages. The myeloid cell compartment represents an important target of HIV-1 infection. While myeloid reservoirs may be more difficult to measure than CD4+ T cell reservoirs, they are long-lived, contribute to viral persistence, and, unless specifically targeted, will prevent an HIV-1 cure.

Identification of CX3CR1+ mononuclear phagocyte subsets involved in HIV-1 and SIV colorectal transmission

The difficulty to unambiguously identify the various subsets of mononuclear phagocytes (MNPs) of the intestinal lamina propria has hindered our understanding of the initial events occurring after mucosal exposure to HIV-1.

Here, we compared the composition and function of MNP subsets at steady-state and following ex vivo and in vivo viral exposure in human and macaque colorectal tissues.

Combined evaluation of CD11c, CD64, CD103, and CX3CR1 expression allowed to differentiate lamina propria MNPs subsets common to both species. Among them, CD11c⁺ CX3CR1⁺ cells expressing CCR5 migrated inside the epithelium following ex vivo and in vivo exposure of colonic tissue to HIV-1 or SIV. In addition, the predominant population of CX3CR1^high macrophages present at steady-state partially shifted to CX3CR1^low macrophages as early as three days following in vivo SIV rectal challenge of macaques.

Our analysis identifies CX3CR1⁺ MNPs as novel players in the early events of HIV-1 and SIV colorectal transmission.

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Human and macaque intestinal MNPs show similar phenotype, localization, and function

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CX3CR1⁺ MNPs migrate inside the intestinal epithelium to sample HIV/SIV

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SIV infection alters the balance between CX3CR1^high and CX3CR1^low Mφs

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CX3CR1⁺ Mφs contribute to the breakdown of the intestinal barrier in HIV/SIV infection

Cell Cycle Regulation in Macrophages and Susceptibility to HIV-1

Macrophages are the first line of defence against invading pathogens. They play a crucial role in immunity but also in regeneration and homeostasis. Their remarkable plasticity in their phenotypes and function provides them with the ability to quickly respond to environmental changes and infection. Recent work shows that macrophages undergo cell cycle transition from a G0/terminally differentiated state to a G1 state. This G0-to-G1 transition presents a window of opportunity for HIV-1 infection. Macrophages are an important target for HIV-1 but express high levels of the deoxynucleotide-triphosphate hydrolase SAMHD1, which restricts viral DNA synthesis by decreasing levels of dNTPs. While the G0 state is non-permissive to HIV-1 infection, a G1 state is very permissive to HIV-1 infection. This is because macrophages in a G1 state switch off the antiviral restriction factor SAMHD1 by phosphorylation, thereby allowing productive HIV-1 infection. Here, we explore the macrophage cell cycle and the interplay between its regulation and permissivity to HIV-1 infection.

Comparative Assessment of Small and Large Intestine Biopsies for Ex Vivo HIV-1 Pathogenesis Studies

Ex vivo mucosal explants have become a mainstay of HIV-1 studies using human tissue. In this study, we examine the baseline phenotypic and virologic differences between biopsies derived from the small intestine (SI) and large intestine (LI) for use in ex vivo explant studies. To do this, we collected endoscopic mucosal biopsies from both SI and LI from the same healthy, HIV-seronegative participants. Mucosal mononuclear cell phenotypes and quantity were compared using flow cytometry. Comparative HIV-1 infectibility of the explants was assessed using an ex vivo explant HIV-1 infection assay. We found that all biopsies had similar numbers of T cells per biopsy. While the percentage of CD4⁺ T cells from SI biopsies expressed significantly more activation markers (CD38, HLA-DR) and HIV coreceptors (CXCR4, CCR5), the absolute numbers of activated CD4⁺ T cells were similar between both sites. LI explants, however, supported more efficient HIV-1 infection, as evidenced by earlier rise in p24 accumulation and greater percent of infected explants at limiting infectious doses. These results suggest that explants from LI biopsies support more efficient HIV-1 infection than SI biopsies, despite similar numbers of available, activated HIV-1 target cells. These findings highlight important differences in LI and SI explants, which must be considered in designing and interpreting ex vivo HIV-1 infection studies, and suggest that factors within the tissue other than target cell number and activation state may play a role in regulating HIV-1 infection.

Differentiating Immune Cell Targets in Gut-Associated Lymphoid Tissue for HIV Cure

The single greatest challenge to an HIV cure is the persistence of latently infected cells containing inducible, replication-competent proviral genomes, which constitute only a small fraction of total or infected cells in the body. Although resting CD4+ T cells in the blood are a well-known source of viral rebound, more than 90% of the body's lymphocytes reside elsewhere. Many are in gut tissue, where HIV DNA levels per million CD4+ T cells are considerably higher than in the blood. Despite the significant contribution of gut tissue to viral replication and persistence, little is known about the cell types that support persistence of HIV in the gut; importantly, T cells in the gut have phenotypic, functional, and survival properties that are distinct from T cells in other tissues. The mechanisms by which latency is established and maintained will likely depend on the location and cytokine milieu surrounding the latently infected cells in each compartment. Therefore, successful HIV cure strategies require identification and characterization of the exact cell types that support viral persistence, particularly in the gut. In this review, we describe the seeding of the latent HIV reservoir in the gut mucosa; highlight the evidence for compartmentalization and depletion of T cells; summarize the immunologic consequences of HIV infection within the gut milieu; propose how the damaged gut environment may promote the latent HIV reservoir; and explore several immune cell targets in the gut and their place on the path toward HIV cure.

Gastrointestinal tract and the mucosal macrophage reservoir in HIV infection

The gastrointestinal tract (GIT) is a primary site for human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infection, replication, and dissemination. After an initial explosive phase of infection, HIV establishes latency. In addition to CD4 T cells, macrophages are readily infected, which can persist for long periods of time. Though macrophages at various systemic sites are infected, those present in the GIT constitute a major cellular reservoir due to the abundance of these cells at mucosal sites. Here, we review some of the important findings regarding what is known about the macrophage reservoir in the gut and explore potential approaches being pursued in the field to reduce this reservoir. The development of strategies that can lead to a functional cure will need to incorporate approaches that can eradicate the macrophage reservoir in the GIT.

Interactions between HIV-1 and mucosal cells in the female reproductive tract

Worldwide, the heterosexual route is the prevalent mode of HIV-1 transmission, and the female reproductive tract accounts for approximately 40% of all HIV-1 transmissions. HIV-1 infection in the female reproductive tract involves three major events: entry through the mucosal epithelium, productive infection in subepithelial mononuclear cells, and delivery to lymph nodes to initiate systemic infection. Here, we provide a focused review of the interaction between HIV-1 and mucosal epithelial cells, lymphocytes, macrophages, and dendritic cells in female genital mucosa. Increased understanding of these interactions could illuminate new approaches for interdicting HIV-1 heterosexual transmission.

HIV-1 infection: the role of the gastrointestinal tract

The intestinal mucosa has an important role as portal of entry during mother-to-child transmission of HIV-1 and during sexual transmission. Tissue morphology and integrity, as well as distribution of relevant cell types within the mucosa, spanning from the oropharynx to the rectum, can greatly influence viral infection, replication, presentation, and persistence. The relative contribution to transmission by cell-associated or cell-free virus is still not defined for the different routes of transmission. Although the main target cells for HIV-1 replication are the CD4+ T lymphocytes, which are rapidly depleted both in the periphery and in the mucosal tissues, dendritic cells, Langerhans' cells, and macrophages are players in each of these processes. The predominant cells involved may differ according to the tract of the gut and the route of transmission. The microenvironment of the intestinal mucosa, including mucus, antibodies, or chemo-cytokines, can as well influence infection and replication of the virus: their role is still under investigation. The understanding of these processes may help in developing efficient prevention strategies.

A comparison of methods for measuring rectal HIV levels suggests that HIV DNA resides in cells other than CD4+ T cells, including myeloid cells

We compared different techniques for measuring gut HIV reservoirs and assessed for HIV in non-CD4 T cells. HIV DNA levels were similar when measured from rectal biopsies and isolated rectal cells, while HIV RNA tended to be higher in rectal cells. HIV DNA levels in total rectal cells were greater than those predicted from levels in sorted CD4 T cells, suggesting a reservoir in non-CD4 T cells, and HIV DNA was detected in sorted myeloid cells (7/7 subjects).

Comprehensive assessment of HIV target cells in the distal human gut suggests increasing HIV susceptibility toward the anus

BACKGROUND

Prevention of rectal HIV transmission is a high-priority goal for vaccines and topical microbicides because a large fraction of HIV transmissions occurs rectally. Yet, little is known about the specific target-cell milieu in the human rectum other than inferences made from the colon.

METHODS

We conducted a comprehensive comparative in situ fluorescence study of HIV target cells (CCR5-expressing T cells, macrophages, and putative dendritic cells) at 4 and 30 cm proximal of the anal canal in 29 healthy individuals, using computerized analysis of digitized combination stains.

RESULTS

Most strikingly, we find that more than 3 times as many CD68 macrophages express the HIV coreceptor CCR5 in the rectum than in the colon (P = 0.0001), and as such rectal macrophages seem biologically closer to the HIV-susceptible CCR5 phenotype in the vagina than the mostly HIV-resistant CCR5 phenotype in the colon. Putative CD209 dendritic cells are generally enriched in the colon compared with the rectum (P = 0.0004), though their CCR5 expression levels are similar in both compartments. CD3 T-cell densities and CCR5 expression levels are comparable in the colon and rectum.

CONCLUSIONS

Our study establishes the target-cell environment for HIV infection in the human distal gut and demonstrates in general terms that the colon and rectum are immunologically distinct anatomical compartments. Greater expression of CCR5 on rectal macrophages suggests that the most distal sections of the gut may be especially vulnerable to HIV infection. Our findings also emphasize that caution should be exercised when extrapolating data obtained from colon tissues to the rectum.

R5 HIV-1 envelope attracts dendritic cells to cross the human intestinal epithelium and sample luminal virions via engagement of the CCR5

The gastrointestinal tract is a principal route of entry and site of persistence of human immunodeficiency virus type 1 (HIV-1). The intestinal mucosa, being rich of cells that are the main target of the virus, represents a primary site of viral replication and CD4⁺ T-cell depletion. Here, we show both in vitro and ex vivo that HIV-1 of R5 but not X4 phenotype is capable of selectively triggering dendritic cells (DCs) to migrate within 30 min between intestinal epithelial cells to sample virions and transfer infection to target cells. The engagement of the chemokine receptor 5 on DCs and the viral envelope, regardless of the genetic subtype, drive DC migration. Viruses penetrating through transient opening of the tight junctions likely create a paracellular gradient to attract DCs. The formation of junctions with epithelial cells may initiate a haptotactic process of DCs and at the same time favour cell-to-cell viral transmission. Our findings indicate that HIV-1 translocation across the intestinal mucosa occurs through the selective engagement of DCs by R5 viruses, and may guide the design of new prevention strategies.

Stromal down-regulation of macrophage CD4/CCR5 expression and NF-κB activation mediates HIV-1 non-permissiveness in intestinal macrophages

Tissue macrophages are derived exclusively from blood monocytes, which as monocyte-derived macrophages support HIV-1 replication. However, among human tissue macrophages only intestinal macrophages are non-permissive to HIV-1, suggesting that the unique microenvironment in human intestinal mucosa renders lamina propria macrophages non-permissive to HIV-1. We investigated this hypothesis using blood monocytes and intestinal extracellular matrix (stroma)-conditioned media (S-CM) to model the exposure of newly recruited monocytes and resident macrophages to lamina propria stroma, where the cells take up residence in the intestinal mucosa. Exposure of monocytes to S-CM blocked up-regulation of CD4 and CCR5 expression during monocyte differentiation into macrophages and inhibited productive HIV-1 infection in differentiated macrophages. Importantly, exposure of monocyte-derived macrophages simultaneously to S-CM and HIV-1 also inhibited viral replication, and sorted CD4⁺ intestinal macrophages, a proportion of which expressed CCR5⁺, did not support HIV-1 replication, indicating that the non-permissiveness to HIV-1 was not due to reduced receptor expression alone. Consistent with this conclusion, S-CM also potently inhibited replication of HIV-1 pseudotyped with vesicular stomatitis virus glycoprotein, which provides CD4/CCR5-independent entry. Neutralization of TGF-β in S-CM and recombinant TGF-β studies showed that stromal TGF-β inhibited macrophage nuclear translocation of NF-κB and HIV-1 replication. Thus, the profound inability of intestinal macrophages to support productive HIV-1 infection is likely the consequence of microenvironmental down-regulation of macrophage HIV-1 receptor/coreceptor expression and NF-κB activation.

Human intestinal macrophages, unlike lymphoid tissue macrophages, brain microglia and genital (vaginal) macrophages, are profoundly incapable of supporting productive HIV-1 infection. Intriguingly, all macrophages are derived exclusively from blood monocytes, which are HIV-1 permissive after differentiation into monocyte-derived macrophages. Therefore, the unique non-permissiveness of intestinal macrophages to HIV-1 must be conferred by the intestinal mucosal microenvironment. Here we report that intestinal stroma potently blocked up-regulation of HIV-1 receptor/coreceptor CD4 and CCR5 expression during monocyte differentiation into macrophages and macrophage nuclear translocation of NF-κB, which is a critical requirement for HIV-1 transcription. These two mechanisms work collaboratively to render intestinal macrophages non-permissive to HIV-1. Harnessing this natural antiviral defense may provide a novel strategy to exploit for the prevention of infection in HIV-1 permissive cells.

HIV-1 and the macrophage

Macrophages and CD4+ T cells are natural target cells for HIV-1, and both cell types contribute to the establishment of the viral reservoir that is responsible for continuous residual virus replication during antiretroviral therapy and viral load rebound upon treatment interruption. Scientific findings that support a critical role for the infected monocyte/macrophage in HIV-1-associated diseases, such as neurological disorders and cardiovascular disease, are accumulating. To prevent or treat these HIV-1-related diseases, we need to halt HIV-1 replication in the macrophage reservoir. This article describes our current knowledge of how monocytes and certain macrophage subsets are able to restrict HIV-1 infection, in addition to what makes macrophages respond less well to current antiretroviral drugs as compared with CD4+ T cells. These insights will help to find novel approaches that can be used to meet this challenge.

Intestinal macrophages and response to microbial encroachment

Macrophages in the gastrointestinal mucosa represent the largest pool of tissue macrophages in the body. In order to maintain mucosal homeostasis, resident intestinal macrophages uniquely do not express the lipopolysaccharide (LPS) co-receptor CD14 or the IgA (CD89) and IgG (CD16, 32, and 64) receptors, yet prominently display Toll-like receptors (TLRs) 3-9. Remarkably, intestinal macrophages also do not produce proinflammatory cytokines in response to TLR ligands, likely because of extracellular matrix (stromal) transforming growth factor-β (TGF-β) dysregulation of nuclear factor (NF)-κB signal proteins and, via Smad signaling, expression of IκBα, thereby inhibiting NF-κB-mediated activities. Thus, in noninflamed mucosa, resident macrophages are inflammation anergic but retain avid scavenger and host defense function, an ideal profile for macrophages in close proximity to gut microbiota. In the event of impaired epithelial integrity during intestinal infection or inflammation, however, blood monocytes also accumulate in the lamina propria and actively pursue invading microorganisms through uptake and degradation of the organism and release of inflammatory mediators. Consequently, resident intestinal macrophages are inflammation adverse, but when the need arises, they receive assistance from newly recruited circulating monocytes.

Inflammation anergy in human intestinal macrophages is due to Smad-induced IkappaBalpha expression and NF-kappaB inactivation

Human intestinal macrophages contribute to tissue homeostasis in noninflamed mucosa through profound down-regulation of pro-inflammatory cytokine release. Here, we show that this down-regulation extends to Toll-like receptor (TLR)-induced cytokine release, as intestinal macrophages expressed TLR3-TLR9 but did not release cytokines in response to TLR-specific ligands. Likely contributing to this unique functional profile, intestinal macrophages expressed markedly down-regulated adapter proteins MyD88 and Toll interleukin receptor 1 domain-containing adapter-inducing interferon beta, which together mediate all TLR MyD88-dependent and -independent NF-kappaB signaling, did not phosphorylate NF-kappaB p65 or Smad-induced IkappaBalpha, and did not translocate NF-kappaB into the nucleus. Importantly, transforming growth factor-beta released from intestinal extracellular matrix (stroma) induced identical down-regulation in the NF-kappaB signaling and function of blood monocytes, the exclusive source of intestinal macrophages. Our findings implicate stromal transforming growth factor-beta-induced dysregulation of NF-kappaB proteins and Smad signaling in the differentiation of pro-inflammatory blood monocytes into noninflammatory intestinal macrophages.

New-generation 5-HT4 receptor agonists: potential for treatment of gastrointestinal motility disorders

IMPORTANCE OF THE FIELD

Gastrointestinal (GI) dysmotility is an important mechanism in functional GI disorders (FGIDs) including constipation, irritable bowel syndrome, functional dyspepsia, and gastroparesis. 5-hydroxytryptamine(4) (5-HT(4)) receptors are targets for the treatment of GI motility disorders. However, older 5-HT(4) receptor agonists had limited clinical success because they were associated with changes in the function of the cardiac HERG potassium channel.

AREAS COVERED IN THIS REVIEW

We conducted a PubMed search using the following key words alone or in combination: 5-HT(4), safety, toxicity, pharmacokinetics, pharmacodynamics, clinical trial, cardiac, hERG, arrhythmia, potassium current, elderly, prucalopride, ATI-7505, and velusetrag (TD-5108), to review mechanisms of action, clinical efficacy, safety and tolerability of three new-generation 5-HT(4) receptor agonists.

WHAT THE READER WILL GAIN

Prucalopride, ATI-7505, and velusetrag (TD-5108) are highly selective, high-affinity 5-HT(4) receptor agonists that are devoid of action on other receptors within their therapeutic range. Their efficacy has been demonstrated in pharmacodynamic studies which demonstrate acceleration of colonic transit and, to a variable degree, in clinical trials that significantly relieve chronic constipation. Currently available evidence shows that the new 5-HT(4) receptor agonists have safe cardiac profiles.

TAKE HOME MESSAGE

New-generation 5-HT(4) receptor agonists and future drugs targeting organ-specific splice variants are promising approaches to treat GI dysmotility, particularly colonic diseases.

The macrophage in HIV-1 infection: from activation to deactivation?

Macrophages play a crucial role in innate and adaptative immunity in response to microorganisms and are an important cellular target during HIV-1 infection. Recently, the heterogeneity of the macrophage population has been highlighted. Classically activated or type 1 macrophages (M1) induced in particular by IFN-γ display a pro-inflammatory profile. The alternatively activated or type 2 macrophages (M2) induced by Th-2 cytokines, such as IL-4 and IL-13 express anti-inflammatory and tissue repair properties. Finally IL-10 has been described as the prototypic cytokine involved in the deactivation of macrophages (dM). Since the capacity of macrophages to support productive HIV-1 infection is known to be modulated by cytokines, this review shows how modulation of macrophage activation by cytokines impacts the capacity to support productive HIV-1 infection. Based on the activation status of macrophages we propose a model starting with M1 classically activated macrophages with accelerated formation of viral reservoirs in a context of Th1 and proinflammatory cytokines. Then IL-4/IL-13 alternatively activated M2 macrophages will enter into the game that will stop the expansion of the HIV-1 reservoir. Finally IL-10 deactivation of macrophages will lead to immune failure observed at the very late stages of the HIV-1 disease.

Host hindrance to HIV-1 replication in monocytes and macrophages

Monocytes and macrophages are targets of HIV-1 infection and play critical roles in multiple aspects of viral pathogenesis. HIV-1 can replicate in blood monocytes, although only a minor proportion of circulating monocytes harbor viral DNA. Resident macrophages in tissues can be infected and function as viral reservoirs. However, their susceptibility to infection, and their capacity to actively replicate the virus, varies greatly depending on the tissue localization and cytokine environment. The susceptibility of monocytes to HIV-1 infection in vitro depends on their differentiation status. Monocytes are refractory to infection and become permissive upon differentiation into macrophages. In addition, the capacity of monocyte-derived macrophages to sustain viral replication varies between individuals. Host determinants regulate HIV-1 replication in monocytes and macrophages, limiting several steps of the viral life-cycle, from viral entry to virus release. Some host factors responsible for HIV-1 restriction are shared with T lymphocytes, but several anti-viral mechanisms are specific to either monocytes or macrophages. Whilst a number of these mechanisms have been identified in monocytes or in monocyte-derived macrophages in vitro, some of them have also been implicated in the regulation of HIV-1 infection in vivo, in particular in the brain and the lung where macrophages are the main cell type infected by HIV-1. This review focuses on cellular factors that have been reported to interfere with HIV-1 infection in monocytes and macrophages, and examines the evidences supporting their role in vivo, highlighting unique aspects of HIV-1 restriction in these two cell types.

Role of the gastrointestinal tract in establishing infection in primates and humans

PURPOSE OF REVIEW

Here we present recent studies examining the gastrointestinal tract during primary HIV and simian immunodeficiency virus infections and emphasizing the onset of severe pathologic processes that are not adequately reflected in peripheral blood. We discuss these findings and hypotheses relating to the role of the gastrointestinal tract in HIV-1 pathogenesis.

RECENT FINDINGS

High levels of viral replication in the gastrointestinal mucosa during primary HIV and simian immunodeficiency virus infections lead to severe depletion of effector memory CD4 T cells coinciding with increased immune activation and mucosal damage. Viral reservoirs established at this stage appear to be persistent over the course of infection and during therapy. In the simian immunodeficiency virus model of AIDS, onset of the impaired intestinal epithelial barrier function and renewal was observed during primary viral infection. Dysfunction of the mucosal immune system and the epithelial barrier may contribute to viral persistence and impaired responses to microbial pathogens in infected individuals.

SUMMARY

A better understanding of the impact of HIV infection on the mucosal immune system may help in the development of newer preventive and therapeutic strategies directed against the virus.

Macrophages in vaginal but not intestinal mucosa are monocyte-like and permissive to human immunodeficiency virus type 1 infection

Mucosal surfaces play a major role in human immunodeficiency virus type 1 (HIV-1) transmission and pathogenesis, and yet the role of lamina propria macrophages in mucosal HIV-1 infection has received little investigative attention. We report here that vaginal and intestinal macrophages display distinct phenotype and HIV-1 permissiveness profiles. Vaginal macrophages expressed the innate response receptors CD14, CD89, CD16, CD32, and CD64 and the HIV-1 receptor/coreceptors CD4, CCR5, and CXCR4, similar to monocytes. Consistent with this phenotype, green fluorescent protein-tagged R5 HIV-1 entered macrophages in explanted vaginal mucosa as early as 30 min after inoculation of virus onto the epithelium, and purified vaginal macrophages supported substantial levels of HIV-1 replication by a panel of highly macrophage-tropic R5 viruses. In sharp contrast, intestinal macrophages expressed no detectable, or very low levels of, innate response receptors and HIV-1 receptor/coreceptors and did not support HIV-1 replication, although virus occasionally entered macrophages in intestinal tissue explants. Thus, vaginal, but not intestinal, macrophages are monocyte-like and permissive to R5 HIV-1 after the virus has translocated across the epithelium. These findings suggest that genital and gut macrophages have different roles in mucosal HIV-1 pathogenesis and that vaginal macrophages play a previously underappreciated but potentially important role in mucosal HIV-1 infection in the female genital tract.

Cytomegalovirus blocks intestinal stroma-induced down-regulation of macrophage HIV-1 infection

Intestinal macrophages, unlike macrophages from other tissues, do not support HIV-1 infection or produce proinflammatory cytokines. In vitro studies suggest this unique, functional phenotype is a result of the exposure of newly recruited blood monocytes to intestinal stromal products. However, in AIDS-related CMV colitis, mucosal macrophages express HIV-1 and proinflammatory cytokines. Therefore, we investigated the mechanism by which CMV confers permissiveness to HIV-1 and cytokine production on intestinal macrophages. We show that intestinal stroma-conditioned media (S-CM) down-regulated monocyte-derived macrophage infection by HIV-1 (pseudotyped with YU2 envelope or vesicular stomatitis virus glycoprotein) and production of TNF-alpha, but preinfection of the cells with CMV reversed this down-regulation, enhancing HIV-1 infection, p24 production, and TNF-alpha release. The ability of CMV to reverse S-CM down-regulation of macrophage HIV-1 infection was blocked by anti-TNF-alpha antibodies and over-ridden by exogenous TNF-alpha. Immunohistochemical analysis of monocyte-derived macrophages exposed to CMV and HIV-1 (YU2 pseudotype) revealed that the cells infrequently contained CMV and HIV-1 viral proteins. In addition, analysis of colon tissue sections from HIV-1-infected patients with CMV colitis showed that some macrophage-like cells contained CMV and TNF-alpha proteins, others contained HIV-1 and TNF-alpha proteins, but cells infrequently contained CMV and HIV-1 proteins. These results indicate that CMV blocks stromal product inhibition of HIV-1 infection in macrophages, and this inhibition is mediated, at least in part, by CMV-induced TNF-alpha acting in trans to enhance HIV-1 infection.

Intestinal macrophages: unique effector cells of the innate immune system

The gastrointestinal mucosa is the largest reservoir of macrophages in the body. These important effector cells are derived from blood monocytes that are recruited to the lamina propria by endogenous chemoattractants in the non-inflamed mucosa and by inflammatory chemokines and bacterial products during inflammation. In the non-inflamed mucosa, newly recruited pro-inflammatory monocytes are exposed to lamina propria stromal (extracellular matrix) factors that induce phenotypic and functional differentiation into non-inflammatory macrophages. As a consequence of this differentiation, resident lamina propria macrophages are strikingly downregulated for the expression of innate response receptors, such as the receptors for lipopolysaccharide, immunoglobulin G (IgG), and IgA, and the production of pro-inflammatory cytokines, including interleukin-1 (IL-1), IL-6, IL-8, and tumor necrosis factor-alpha. Despite downregulated pro-inflammatory function, strong phagocytic and bactericidal activities remain intact. Thus, in the non-inflamed intestinal mucosa, lamina propria macrophages are non-inflammatory but retain avid scavenger and host defense functions, a unique but ideal phenotype and functional profile for effector cells in close proximity to immunostimulatory microorganisms and products.

Human intestinal macrophages display profound inflammatory anergy despite avid phagocytic and bacteriocidal activity

Intestinal macrophages, which are thought to orchestrate mucosal inflammatory responses, have received little investigative attention compared with macrophages from other tissues. Here we show that human intestinal macrophages do not express innate response receptors, including the receptors for LPS (CD14), Fcalpha (CD89), Fcgamma (CD64, CD32, CD16), CR3 (CD11b/CD18), and CR4 (CD11c/CD18); the growth factor receptors IL-2 (CD25) and IL-3 (CD123); and the integrin LFA-1 (CD11a/CD18). Moreover, resident intestinal macrophages also do not produce proinflammatory cytokines, including IL-1, IL-6, IL-10, IL-12, RANTES, TGF-beta, and TNF-alpha, in response to an array of inflammatory stimuli but retain avid phagocytic and bacteriocidal activity. Thus, intestinal macrophages are markedly distinct in phenotype and function from blood monocytes, although intestinal macrophages are derived from blood monocytes. To explain this paradox, we show that intestinal stromal cell-derived products downregulate both monocyte receptor expression and, via TGF-beta, cytokine production but not phagocytic or bacteriocidal activity, eliciting the phenotype and functional profile of intestinal macrophages. These findings indicate a mechanism in which blood monocytes recruited to the intestinal mucosa retain avid scavenger and host defense functions but acquire profound "inflammatory anergy," thereby promoting the absence of inflammation characteristic of normal intestinal mucosa despite the close proximity of immunostimulatory bacteria.

Human intestinal macrophages display profound inflammatory anergy despite avid phagocytic and bacteriocidal activity

Intestinal macrophages, which are thought to orchestrate mucosal inflammatory responses, have received little investigative attention compared with macrophages from other tissues. Here we show that human intestinal macrophages do not express innate response receptors, including the receptors for LPS (CD14), Fcalpha (CD89), Fcgamma (CD64, CD32, CD16), CR3 (CD11b/CD18), and CR4 (CD11c/CD18); the growth factor receptors IL-2 (CD25) and IL-3 (CD123); and the integrin LFA-1 (CD11a/CD18). Moreover, resident intestinal macrophages also do not produce proinflammatory cytokines, including IL-1, IL-6, IL-10, IL-12, RANTES, TGF-beta, and TNF-alpha, in response to an array of inflammatory stimuli but retain avid phagocytic and bacteriocidal activity. Thus, intestinal macrophages are markedly distinct in phenotype and function from blood monocytes, although intestinal macrophages are derived from blood monocytes. To explain this paradox, we show that intestinal stromal cell-derived products downregulate both monocyte receptor expression and, via TGF-beta, cytokine production but not phagocytic or bacteriocidal activity, eliciting the phenotype and functional profile of intestinal macrophages. These findings indicate a mechanism in which blood monocytes recruited to the intestinal mucosa retain avid scavenger and host defense functions but acquire profound "inflammatory anergy," thereby promoting the absence of inflammation characteristic of normal intestinal mucosa despite the close proximity of immunostimulatory bacteria.

Macrophage HIV-1 infection and the gastrointestinal tract reservoir

Excluding parenteral transmissions, virtually all vertical and homosexual transmission of human immunodeficiency virus type 1 (HIV-1) occurs via the gastrointestinal tract. Cellular routes implicated in the translocation of virus across the epithelium include M cells, dendritic cells, and epithelial cells. Intestinal epithelial cells express CCR5 and can selectively transfer CCR5-tropic HIV-1, the phenotype of the majority of transmitted viruses. In the lamina propria, virus encounters the largest reservoir of mononuclear cells in the body. Surprisingly, lamina propria lymphocytes, not macrophages, express CCR5 and CXCR4 and support HIV-1 replication, implicating intestinal lymphocytes as the initial target cell in the intestinal mucosa. From the mucosa, virus is disseminated to systemic sites, followed by profound depletion of CD4+ T cells, first in the intestinal lamina propria and subsequently in the blood. As mucosal and circulating CD4+ T cells are depleted, monocytes and macrophages assume an increasingly important role as target and reservoir cells for HIV-1. Blood monocytes, including HIV-1-infected cells, are recruited to the mucosa, where they differentiate into lamina propria macrophages in the presence of stroma-derived factors. Although the prevalence of HIV-1-infected macrophages in the mucosa is low (0.06% of lamina propria mononuclear cells), the extraordinary size of the gastrointestinal mucosa imparts to intestinal macrophages a prominent role as a HIV-1 reservoir. Elucidating the immunobiology of mucosal HIV-1 infection is critical for understanding disease pathogenesis and ultimately for devising an effective mucosal HIV-1 vaccine.

Internal anal sphincter achalasia in children: clinical characteristics and treatment with Clostridium botulinum toxin

OBJECTIVES

To describe the clinical characteristics of children with internal anal sphincter (IAS) achalasia and to evaluate the benefit of intrasphincteric injection of Clostridium botulinum toxin.

METHODS

Retrospective review of the medical records of 20 patients (8 male, mean 5.8 +/- 4.2 years) with severe chronic constipation and IAS achalasia. Each patient received four-quadrant, intrasphincteric injections of botulinum toxin at a dose of 15-25U per quadrant. Patients were reassessed 4 weeks-18 months after injection. We compared the clinical characteristics of these patients to 20 consecutive children (14 male, mean age 8.1 + 4.6 y) with functional constipation (control group).

RESULTS

The children with IAS achalasia had earlier onset of symptoms, less fecal soiling, and less withholding behavior than the control children. Response to botulinum injection was rated excellent by the parents in 60% and by the physician in 35% of children. There was wide individual variability in the frequency of defecation after therapy. Duration of response ranged from 1 week to 18 months.

CONCLUSIONS

Children with IAS achalasia have clinical characteristics differentiating them from children with functional constipation. Intra-anal injection of botulinum toxin is a safe and effective short-term treatment for these children.

Fcgamma receptor-mediated suppression of human immunodeficiency virus type 1 replication in primary human macrophages

Permissiveness of monocytes and macrophages to human immunodeficiency virus (HIV) infection is modulated by various stimuli. In this study we demonstrate that stimulation of primary monocytes and monocyte-derived macrophages (MDM) through the receptors for the Fc portion of immunoglobulin G (IgG) (FcgammaR) inhibits HIV type 1 (HIV-1) replication. Viral p24 production was decreased by 1.5 to 3 log units in MDM infected with both R5 and X4 HIV-1 strains upon stimulation by immobilized IgG but not upon stimulation by soluble IgG or by F(ab')(2) IgG fragments. Although MDM activation by immobilized IgG induced high levels of macrophage-derived chemokine secretion as well as a sustained down-regulation of CD4 and a transient decrease in CCR5 expression, these factors did not appear to play a major role in the suppression of HIV-1 replication. Single-cycle infection of FcgammaR-stimulated MDM with HIV-1 virions pseudotyped with either HIV-1 R5 or vesicular stomatitis virus G envelopes was inhibited, suggesting a postentry restriction of viral replication. PCR analyses of HIV-1 DNA intermediate replication forms suggested that reverse transcription is not affected by stimulation with immobilized human IgG, at least during the first replication cycle. The accumulation of PCR products corresponding to nuclear unintegrated two-long-terminal-repeat circles and the relative decrease of integrated HIV-1 DNA signals suggest an inhibition of proviral integration. Our data, showing that FcgammaR-mediated activation of MDM is a potent mechanism of HIV-1 suppression, raise the possibility that FcgammaR cross-linking by immune complexes may contribute to the control of viral replication in macrophages.

Expression of chemokine receptors in the feline reproductive tract and large intestine

Feline immunodeficiency virus (FIV) is a lentivirus that causes feline acquired immunodeficiency syndrome. Infection can be transmitted experimentally via the vagina and rectum, making the cat a useful model for human immunodeficiency virus (HIV) infection. Some strains of FIV use the CXCR4 chemokine receptor in vitro to gain entry to feline cell lines, thymocytes and peripheral blood leucocytes (PBLs). In this study, the tissue expression of messenger ribonucleic acid (mRNA) encoding the CCR3, CXCR4 and CCR5 receptors was examined by reverse transcriptase polymerase chain reaction (RT-PCR). mRNA encoding each receptor was expressed by two feline T-cell lines (Mya-1 and FeTJ), a feline kidney fibroblast cell line (FKCU) and PBLs. Mesenteric lymph node, colon, rectum, uterus, cervix and vagina all expressed mRNA for CXCR4 and CCR5 whilst only lymph node expressed CCR3 mRNA. In order to locate this receptor mRNA expression, in-situ hybridization studies were performed with DNA probes specific for the chemokine receptor mRNAs. CCR5 and CXCR4 receptor mRNA was expressed by epithelial cells and some lamina propria cells of the colon and rectum. Epithelial cell expression of chemokine receptor mRNA was reduced in intensity towards the base of the crypts. Expression of CXCR4 receptor was also demonstrated immunohistochemically on some lamina propria and intraepithelial cells. The expression of these receptor molecules may be important in mucosal infection with FIV.

Primary intestinal epithelial cells selectively transfer R5 HIV-1 to CCR5+ cells

The upper gastrointestinal tract is a principal route of HIV-1 entry in vertical transmission and after oral-genital contact. The phenotype of the newly acquired virus is predominantly R5 (CCR5-tropic) and not X4 (CXCR4-tropic), although both R5 and X4 viruses are frequently inoculated onto the mucosa. Here we show that primary intestinal (jejunal) epithelial cells express galactosylceramide, an alternative primary receptor for HIV-1, and CCR5 but not CXCR4. Moreover, we show that intestinal epithelial cells transfer R5, but not X4, viruses to CCR5+ indicator cells, which can efficiently replicate and amplify virus expression. Transfer was remarkably efficient and was not inhibited by the fusion blocker T-20, but was substantially reduced by colchicine and low (4 degrees C) temperature, suggesting endocytotic uptake and microtubule-dependent transcytosis of HIV-1. Our finding that CCR5+ intestinal epithelial cells select and transfer exclusively R5 viruses indicates a mechanism for the selective transmission of R5 HIV-1 in primary infection acquired through the upper gastrointestinal tract.

Emerging treatments for irritable bowel syndrome

Irritable bowel syndrome (IBS) is a functional GI disorder that is associated with abdominal discomfort and altered bowel habits. It accounts for up to 28% of patients presenting to a gastroenterology practice and poses a significant personal, societal and economic burden internationally. The Manning, Rome I and Rome II criteria were developed to identify appropriate IBS patients for entry into IBS studies in a consistent manner. Refinements in the understanding of the physiology of the enteric nervous system (ENS), which controls motility, secretion and sensation, provided the basis for our comprehension of the pathophysiology of IBS. Visceral hypersensitivity and neurotransmitter imbalance currently receive the most attention as possible mechanisms of IBS. This article outlines conventional treatments and reviews the data on emerging and experimental therapies for IBS. Emerging therapies for IBS using 5-HT mediation include 5-HT(3) antagonists, such as ondasetron, granisetron and alosetron, as well as 5-HT(4) agonists such as tegaserod and prucalopride. In addition to opioid agonists (e.g. fedotozine) several other drugs that act on other ENS receptors are being studied. In spite of significant progress in IBS research, these emerging therapies require more studies before they can be utilised as clinical treatments.

Identification of shared populations of human immunodeficiency virus type 1 infecting microglia and tissue macrophages outside the central nervous system

Infection of microglia and other cells of the macrophage/monocyte lineage in the central nervous system (CNS) by human immunodeficiency virus type I (HIV-1) underlies the development of giant cell encephalitis (GCE). It is currently unknown whether GCE depends on the emergence of virus populations specifically adapted to replicate in cells of the monocyte/macrophage lineage and whether this also leads to the specific targeting of macrophages in other nonlymphoid tissues. Autopsy samples from lymph node, brain (frontal region), lung, and full-thickness colon sections were obtained from nine study subjects with GCE and from nine without. The two groups showed no significant differences in CD4 counts, disease progression, or treatment history before death. Genetic relatedness between variants recovered from lymph node and nonlymphoid tissues was assessed by sequence comparison of V3 and p17(gag) regions using a newly developed method that scores the sample composition at successive nodes in a neighbor-joining tree. The association index enabled objective, numerical comparisons on the degree of tissue compartmentalization to be made. High proviral loads and p24 antigen expression in the brain were confined to the nine individuals with GCE. GCE was also associated with significantly higher proviral loads in colon samples (median of the GCE(+) group: 1,010 copies/10(6) cells; median of GCE(-) group, 10/10(6) cells; P = 0.006). In contrast, there were no significant differences in proviral load between the GCE(+) and GCE(-) groups in lymph node or lung samples, where HIV infection was manifested predominantly by infiltrates of lymphoid cells. V3 sequences from brain samples of individuals with GCE showed the greatest compartmentalization from those of lymph node, although samples from other tissues, particularly the colon, frequently contained variants phylogenetically related to those found in brain. The existence of shared, distinct populations of HIV specifically distributed in cells of the monocyte/macrophage lineage was further indicated by immunocytochemical detection of CD68(+), multinucleated giant cells expressing p24 antigen in samples of lung and colon in two individuals with GCE. This study provides the basis for future investigation of possible phenotypic similarities that underline the shared distributions of HIV variants infecting microglia and tissue macrophages outside the CNS.

Intestinal macrophages lack CD14 and CD89 and consequently are down-regulated for LPS- and IgA-mediated activities

The intestinal mucosa normally displays minimal inflammation despite the close proximity between mucosal macrophages and lumenal bacteria. Macrophages interact with bacteria and their products through CD14, a surface receptor involved in the response to LPS, and CD89, the receptor for IgA (FcalphaR). Here we show that resident macrophages isolated from normal human intestine lack CD14 and CD89. The absence of CD14 and CD89 was not due to the isolation procedure or mucosal cell products, but was evident at the transcriptional level, as the macrophages expressed neither CD14- nor CD89-specific mRNAs, but did express Toll-like receptor 2 and 4 transcripts. Consistent with their CD14(-) phenotype, lamina propria macrophages displayed markedly reduced LPS-induced cytokine production and LPS-enhanced phagocytosis. In addition, IgA-enhanced phagocytosis was sharply reduced in lamina propria macrophages. Thus, the absence of CD14 and CD89 on resident intestinal macrophages, due to down-regulated gene transcription, causes down-modulated LPS- and IgA-mediated functions and probably contributes to the low level of inflammation in normal human intestinal mucosa.

Emerging concepts in gastrointestinal aspects of HIV-1 pathogenesis and management

GASTROENTROLOGY 2001;120:607-621

New therapies for functional bowel diseases

During recent years, a greater understanding of the pathophysiology of functional bowel disease, and a surge of interest in this challenge among pharmacologists, basic scientists, and clinical investigators, have led to novel insights and promising therapies. The evidence to support current therapies in nonculcer dyspepsia and irritable bowel syndrome (IBS) is reviewed, and some of the novel therapeutic approaches on the threshold of clinical application are described.

Mucosal events in the pathogenesis of human immunodeficiency virus type 1 infection

The interaction between human immunodeficiency virus type 1 (HIV-1) and primary mucosal cells isolated from normal human small intestine was investigated. Purified primary intestinal epithelial cells could transport cell-free HIV-1 to mononuclear cells, although the epithelial cells did not support viral replication. An unexpected finding was that primary intestinal macrophages were markedly less permissive to HIV-1 than were blood monocytes. The reduced permissiveness appeared to be due to the near absence of surface CCR5 on resident intestinal macrophages. Surface CCR5 could be up-regulated on the monocytes but not the intestinal macrophages by HIV-1 and gp120. Impaired permissiveness of intestinal macrophages to HIV-1 may play an important role in the low prevalence of HIV-1 mRNA-expressing macrophages in the lamina propria during HIV-1 infection in vivo. Characterization of the biologic properties of HIV-1 transport and infection in primary mucosal cells will be key to elucidating the pivotal role of mucosal surfaces in HIV-1 disease.