Persistent and generalized lymphadenopathy: a lesion of follicular dendritic cells? An immunohistologic and ultrastructural study

HIV-1 infection is associated with depletion of germinal center B cells and a decrease in IgA + plasma cells in the GI tract

Gastrointestinal (GI) B cells and plasma cells (PCs), critical to mucosal homeostasis, play an important role in the host response to HIV-1 infection. Here, high resolution mapping of human B cells and PCs from colon and ileum during both viremic and suppressed HIV-1 infection identified a significant reduction in germinal center (GC) B cells and Follicular Dendritic Cells (FDCs) during HIV-1 viremia. Further, IgA + PCs, the major cellular output of intestinal GCs were significantly reduced during viremic HIV-1 infection. PC-associated transcriptional perturbations, including type I interferon signaling persisted in antiretroviral therapy (ART) treated individuals, suggesting ongoing disruption of the intestinal immune milieu during ART. GI humoral immune perturbations associated with changes in intestinal microbiome composition and systemic inflammation. Herein, we highlight a key immune defect in the GI mucosa due to HIV-1 viremia, with major implications.

One Sentence Summary

Major perturbations in intestinal GC dynamics in viremic HIV-1 infection relate to reduced IgA + plasma cells, systemic inflammation and microbiota changes.

Differentiation and Function of Follicular CD8 T Cells During Human Immunodeficiency Virus Infection

The combination antiretroviral therapeutic (cART) regime effectively suppresses human immunodeficiency virus (HIV) replication and prevents progression to acquired immunodeficiency diseases. However, cART is not a cure, and viral rebound will occur immediately after treatment is interrupted largely due to the long-term presence of an HIV reservoir that is composed of latently infected target cells that maintain a quiescent state or persistently produce infectious viruses. CD4 T cells that reside in B-cell follicles within lymphoid tissues, called follicular helper T cells (TFH), have been identified as a major HIV reservoir. Due to their specialized anatomical structure, HIV-specific CD8 T cells are largely insulated from this TFH reservoir. It is increasingly clear that the elimination of TFH reservoirs is a key step toward a functional cure for HIV infection. Recently, several studies have suggested that a fraction of HIV-specific CD8 T cells can differentiate into a CXCR5-expressing subset, which are able to migrate into B-cell follicles and inhibit viral replication. In this review, we discuss the differentiation and functions of this newly identified CD8 T-cell subset and propose potential strategies for purging TFH HIV reservoirs by utilizing this unique population.

The B-Cell Follicle in HIV Infection: Barrier to a Cure

The majority of HIV replication occurs in secondary lymphoid organs (SLOs) such as the spleen, lymph nodes, and gut-associated lymphoid tissue. Within SLOs, HIV RNA⁺ cells are concentrated in the B-cell follicle during chronic untreated infection, and emerging data suggest that they are a major source of replication in treated disease as well. The concentration of HIV RNA⁺ cells in the B-cell follicle is mediated by several factors. Follicular CD4⁺ T-cell subsets including T-follicular helper cells and T-follicular regulatory cells are significantly more permissive to HIV than extrafollicular subsets. The B cell follicle also contains a large reservoir of extracellular HIV virions, which accumulate on the surface of follicular dendritic cells (FDCs) in germinal centers. FDC-bound HIV virions remain infectious even in the presence of neutralizing antibodies and can persist for months or even years. Moreover, the B-cell follicle is semi-immune privileged from CTL control. Frequencies of HIV- and SIV-specific CTL are lower in B-cell follicles compared to extrafollicular regions as the majority of CTL do not express the follicular homing receptor CXCR5. Additionally, CTL in the B-cell follicle may be less functional than extrafollicular CTL as many exhibit the recently described CD8 T follicular regulatory phenotype. Other factors may also contribute to the follicular concentration of HIV RNA⁺ cells. Notably, the contribution of NK cells and γδ T cells to control and/or persistence of HIV RNA⁺ cells in secondary lymphoid tissue remains poorly characterized. As HIV research moves increasingly toward the development of cure strategies, a greater understanding of the barriers to control of HIV infection in B-cell follicles is critical. Although no strategy has as of yet proven to be effective, a range of novel therapies to address these barriers are currently being investigated including genetically engineered CTL or chimeric antigen receptor T cells that express the follicular homing molecule CXCR5, treatment with IL-15 or an IL-15 superagonist, use of bispecific antibodies to harness the killing power of the follicular CD8⁺ T cell population, and disruption of the follicle through treatments such as rituximab.

Pathobiology of HIV/SIV-associated changes in secondary lymphoid tissues

Acquired immunodeficiency syndrome (AIDS) is principally a disease of lymphoid tissues (LTs), due to the fact that the main target cell of human immunodeficiency virus (HIV) is the CD4(+) T lymphocyte that primarily resides within organs of the immune system. The impact of HIV infection on secondary LTs, in particular lymph nodes, is critical to delineate, as these immune organs are the principal sites for initiating and facilitating immune responses and are critical for lymphocyte homeostatic maintenance and survival. The underlying structural elements of LTs, fibroblastic reticular cell (FRC) network, not only form the architectural framework for these organs, but also play in integral role in the production and storage of cytokines needed for T-cell survival. There is an interdependent relationship between the FRC stromal network and CD4(+) T lymphocytes for their survival and maintenance that is progressively disrupted during HIV disease. HIV infection results in profound pathological changes to LTs induced by persistent chronic immune activation and inflammation that leads to progressive collagen deposition and fibrosis disrupting and damaging the important FRC network. In this review, I focus on the process, mechanisms, and the implications of pathological damage to important secondary LTs, combining what we have learned from HIV-infected individuals as well as the invaluable knowledge gained from studies in non-human primate simian immunodeficiency virus infection models.

Isolation of functionally active murine follicular dendritic cells

Biochemical, genetic, and immunological studies of follicular dendritic cells (FDCs) have been hampered by difficulty in obtaining adequate numbers of purified cells in a functional state. To address this obstacle, we enriched FDCs by irradiating mice to destroy most lymphocytes, excised the lymph nodes, and gently digested the nodes with an enzyme cocktail to form single cell suspensions. The FDCs in suspension were selected using the specific mAb FDC-M1 with magnetic cell separation technology. We were able to get nearly a million viable lymph node FDCs per mouse at about 90% purity. When examined under light and transmission electron microscopy, the cytological features were characteristic of FDCs. Furthermore, the cells were able to trap and retain immune complexes and were positive for important phenotypic markers including FDC-M1, CD21/35, CD32, CD40, and CD54. Moreover, the purified FDCs exhibited classical FDC accessory activities including: the ability to co-stimulate B cell proliferation, augment antibody responses induced by mitogens or antigens, maintain B cell viability for weeks, and protect B lymphocytes from anti-FAS induced apoptosis. In short, this combination of methods made it possible to obtain a substantial number of highly enriched functional murine FDCs.

Enfermedad de Castleman multicéntrica en sida. Su relación con el VHH-8 o virus herpes asociado al sarcoma de Kaposi. Estudio de dos casos

Castleman disease is considered a reactive lymphadenopathic picture with two clinical forms: one localized, frequent in immunocompetent patients and another multicenter one that is more characteristic in immunodepressed patients. Two cases of Castleman disease multicenter in HIV positive patients with Kaposi's sarcoma are presented. Both patients have multiple adenopathies, hepatomegaly and symptoms B on diagnosis. One of them had a favorable response to chemotherapy treatment and another died. A review of the concept of multicenter Castleman disease and its pathogenic relationship to human herpes virus 8 (HHV-8) is done.

TGF-beta inhibits Fas-mediated apoptosis of a follicular dendritic cell line by down-regulating the expression of Fas and caspase-8: counteracting role of TGF-beta on TNF sensitization of Fas-mediated apoptosis

Follicular dendritic cells (FDC) constitute the framework of germinal center (GC) in secondary lymphoid follicles, and the integrity of FDC networks is critically affected by cytokines present in the GC. We have previously shown that TNF promotes Fas-mediated apoptosis of HK cells, an established FDC-like cell line, by up-regulating Fas expression. However, in the developing GC, FDC death is not a hallmark of GC despite the presence of TNF and FasL. In this study, we report that TGF-beta inhibits Fas-mediated apoptosis of HK cells by down-regulating the expression of surface Fas and caspase-8. The inhibitory effect of TGF-beta can be observed when HK cells were simultaneously treated with TNF and TGF-beta, indicating that TGF-beta counteracts the effect of TNF in sensitizing cells to Fas-mediated apoptosis. Furthermore, the deprivation of TGF-beta by injecting neutralizing TGF-beta Abs to the SRBC-immunized mice resulted in the sporadic appearance of FDC undergoing apoptosis in the lymphoid follicles, suggesting that TGF-beta functions as a naturally occurring inhibitor that rescues FDCs which are predisposed to apoptosis. Our study documents a novel function of TGF-beta in the maintenance of FDC networks.

Functional effects of TNF-alpha on a human follicular dendritic cell line: persistent NF-kappa B activation and sensitization for Fas-mediated apoptosis

Follicular dendritic cells (FDC) play crucial roles in germinal center (GC) formation and differentiation of GC B cells. FDC functions are influenced by cytokines produced in the GC. Among the GC cytokines, TNF is known to be essential for the formation and maintenance of the FDC network in the GC. We found that TNF is a mitogenic growth factor to an established FDC-like cell line, HK cells. Differing from most cell types which become desensitized to TNF action, HK cells exhibited persistent TNF signaling, as demonstrated by prolonged and biphasic NF-kappaB activation even after 3 days of TNF treatment. As a result, antiapoptotic genes including TNFR-associated factors 1 and 2, and cellular inhibitor of apoptosis proteins 1 and 2 were persistently induced by TNF, leading to the protection against TNF-mediated cell death. However, TNF pretreatment enhanced Fas-mediated apoptosis by up-regulating surface Fas expression in an NF-kappaB-dependent pathway. During the GC responses, proliferation followed by FDC death has not been documented. However, our in vitro results suggest that FDCs proliferate in response to TNF, and die by Fas-mediated apoptosis whose susceptibility is enhanced by TNF, representing a mode of action for TNF in the maintenance of FDC networks by regulating the survival or death of FDC.

Splenic marginal zone atrophy and progressive CD8+ T-cell lymphocytosis in HIV infection: a study of adult post-mortem spleens from Côte d'Ivoire

AIMS

Progressive changes have been reported in lymph nodes in HIV infection, but few accounts describe altered splenic histology at different stages of the disease. Investigation of splenic changes accompanying the progressive CD4+ T-cell depletion that occurs in HIV infection could shed light on normal immunological interactions in this organ. Therefore, we assessed the amount and distribution of lymphoid tissue in spleens from adults with documented early or advanced HIV disease.

METHODS AND RESULTS

Immunohistochemistry was used to study splenic tissue collected in an extensive autopsy survey of HIV+ adults in West Africa. Compared with post-mortem spleens from HIV- West African adults and control UK spleens, those from HIV-infected patients showed severe atrophy of white pulp B- and T-cell compartments. In early and advanced HIV disease, marginal zone atrophy was significant. Peri-arteriolar lymphoid sheaths contained increased numbers of CD8+/CD45RO+ T-cells in advanced HIV disease. In red pulp, early and advanced cases showed a lymphocytosis of CD8+/CD45RO- T-lymphocytes.

CONCLUSIONS

Atrophic changes were more extreme in advanced than early HIV infection. Reduced marginal zone function possibly explains the known predisposition of HIV+ patients to infection by encapsulated bacteria. Possible immunological consequences of these CD8+/CD45RO+ (peri-arteriolar lymphoid sheaths) and CD8+/CD45RO- (red pulp) responses deserve further study. Comparison of West African and UK control spleens indicated that there were no major ethnic differences in spleen structure to prevent extrapolation of our results to European adults.

Dismantling the germinal center: comparing the processes of transformation, regression, and fragmentation of the lymphoid follicle

The pathologic and immunologic features of the formation of the germinal center have been extensively studied. The process of dissolution of the germinal center is not as well understood. Different patterns of germinal-center breakdown are commonly encountered in diagnostic lymph node biopsy specimens and frequently present difficulties in diagnosis. The current immunologic understanding of germinal-center dynamics is reviewed and correlated with the histologic and immunophenotypic features of three broad classes of germinal center dissolution, namely progressive transformation, regression, and follicle fragmentation. The author suggests that these different patterns represent alternate responses to antigenic stimulation. The relationships of progressive transformation to Hodgkin's disease and of follicular regression to Castleman's disease are discussed.

The relative resistance of HIV type 1-infected chimpanzees to AIDS correlates with the maintenance of follicular architecture and the absence of infiltration by CD8+ cytotoxic T lymphocytes

Lymphoid tissues are the focus of critical events in HIV pathogenesis. Persistent and high levels of virus production, extensive trapping of virus particles in germinal centers, and progressive degenerative changes in lymph node architecture are characteristics of progressive HIV-1 infection. Infiltrates of granzyme B- and TIA-expressing CD8+ "cytotoxic" T lymphocytes (CTLs) precede involution of germinal centers in humans who develop AIDS. Similar to humans, HIV-1 infection in chimpanzees is active and persistent. However, in contrast to humans, they remain relatively resistant to AIDS. Lymph node biopsies from chimpanzees infected with HIV-1 or a related chimpanzee lentivirus were studied for the level and pattern of virus expression, changes in lymphoid architecture, CD8+ T cell infiltrates and the presence or absence of CTL markers. In stark contrast to HIV-1-infected humans, lymph nodes from infected chimpanzees had little virus deposition in germinal centers and a paucity of virus-expressing cells. Although some of the lymph nodes examined from infected animals had moderate follicular hyperplasia with infiltrating CD8+ T cells, none had evidence of follicular fragmentation. Most importantly, in marked contrast to infected humans, CD8+ T cells infiltrating the germinal center were negative for the CTL marker granzyme B. This evidence suggests that the infiltration of CD8+ CTLs into the germinal centers of lymph nodes may be a key determinant in AIDS pathogenesis.

Follicular dendritic cells (FDC) in retroviral infection: host/pathogen perspectives

Follicular dendritic cells (FDC) are found in the follicles of virtually all secondary lymphoid tissues. In health, these cells trap and retain antigens (Ag) in the form of immune complexes and preserve them for months in their native conformation. FDC thus serve as a long-term repository of extracellular Ag important for induction and maintenance of memory responses. In retroviral infection, FDC trap and retain large numbers of retroviral particles with profound effects on FDC. FDC-trapped retrovirus induces follicular hyperplasia, and conventional Ag trapped prior to infection are lost and new Ag cannot be trapped. Concomitantly, antibody-forming cells (AFC) specific for Ag lost from FDC decrease followed by loss of specific serum antibody (Ab). Eventually, FDC die and follicular lysis occurs. From the pathogen perspective, binding to FDC is remarkably beneficial, bringing together virus and activated target cells that are highly susceptible to infection. Furthermore, FDC permit HIV to infect surrounding cells even in the presence of a vast excess of neutralizing Ab. Preliminary data suggest that FDC maintain virus infectivity-even when the virus cannot replicate. Thus retrovirus infection monopolizes FDC networks, thereby transforming the FDC Ag repository into a highly infectious retroviral reservoir.

Expression of granzyme B by cytotoxic T lymphocytes in the lymph nodes of HIV-infected patients

During HIV infection the architecture of secondary lymphoid tissues is severely disrupted. In particular the germinal centers, which play a key role in the orchestration of the secondary immune response, undergo gross phenotypic alterations, leading to a complete destruction of the germinal center microenvironment. The precise mechanisms responsible for the lymphoid tissue destruction in HIV infection are still unknown. However, the large influx of CD8+ T lymphocytes suggests an important role for T cell-mediated cytotoxicity. To establish whether the infiltrating CD8+ T lymphocytes are killing competent, we investigated the expression of granzyme B, which is known to be present in the cytotoxic granules of NK cells and "activated" CTLs with cytolytic potential. We observed a 20-fold increase in the percentage of granzyme B-expressing CD8+ T cells in both the germinal center and the interfollicular areas in HIV patients relative to HIV-negative controls. This increase was present in patients with early-stage disease (i.e., absolute CD4+ T cell count > 500/microliters) as well as in patients with intermediate and late-stage disease. Thus, from relatively early stages of HIV infection onward large numbers of killing competent T lymphocytes are present in the lymphoid tissues, a finding that supports the notion that CTL act as mediators of destruction of immune function during HIV infection.

Dark and light zones of germinal centres of the human tonsil: an ultrastructural study with emphasis on heterogeneity of follicular dendritic cells

The cellular composition of the dark and light zones of germinal centres in human tonsils was quantitatively determined by electron microscopy. In addition to the well known germinal-centre B-cells, we defined the cleaved blast as a new distinct cell type in the germinal centre. The dark and the light zones clearly differed in their content of lymphoid and non-lymphoid germinal-centre cells. The dark zone was characterized by higher frequencies of cleaved blasts and small centroblasts, a higher blast-centrocyte ratio and a higher incidence of mitotic figures. In contrast, the light zone had higher frequencies of centrocytes, centroplasmacytoid cells, lymphocytes and follicular dendritic cells (FDC) and an inverted blast-centrocyte ratio. Seven distinct appearances of FDC (FDC.1-FDC.7) could be recognized on the basis of their ultrastructure. The distribution pattern of these subtypes differed between the dark and light zone. The undifferentiated subtypes FDC.2 and FDC.3 predominated in the dark zone. In contrast, the highly differentiated subtypes FDC.4 and FDC.5 were present at a much higher density in the light zone. These findings suggest that the dark and light zones are different compartments with regard to proliferation and differentiation of germinal-centre B-cells and that both compartments have a specific microenvironment influenced by FDC.

Cellular interactions in the germinal center: role of adhesion receptors and significance for the pathogenesis of AIDS and malignant lymphoma

The germinal center forms a specialized microenvironment that is thought to play a key role in the induction of antibody synthesis, affinity maturation of B cells, isotype switching, and memory B-cell formation. Moreover, the germinal center may also be involved in the maintenance of T-cell memory. In this paper we focus on the role of adhesion receptors in cellular interactions in the germinal center, and discuss evidence indicating that these molecules play an important role in regulating B-cell activation and differentiation. Furthermore, we discuss two important diseases involving the germinal center, i.e., HIV infection and malignant lymphoma. In HIV infection, destruction of the FDC network may explain the selective loss of memory cells observed in otherwise asymptomatic patients and is likely to represent a major pathway leading to AIDS. In follicular lymphoma, escape from physiological apoptosis in the germinal center by overexpression of Bcl-2 appears be a major pathogenetic pathway.

Cellular composition of germinal centers in lymph nodes after HIV-1 infection: evidence for an inadequate support of germinal center B lymphocytes by follicular dendritic cells

Germinal centers in lymph nodes with follicular hyperplasia from 15 patients with HIV-1 infection were analyzed by qualitative and quantitative electron microscopical methods and compared with control follicular hyperplasia (FH). Using a pattern recognition method, two main clusters were recognized within the germinal centers of HIV and FH lymph nodes on the basis of the relative frequencies of small centroblast and centrocytes. All FH lymph nodes and 6 HIV-1 lymph nodes (HIV-Clu-1) were placed in cluster 1; 9 HIV-1 lymph nodes (HIV-Clu-2) formed cluster 2. Germinal centers in the HIV-Clu-2 lymph nodes were characterized by a cell composition of predominantly lymphoid blasts and decreased numbers of centrocytes, but without altered numbers of mitotic figures. The frequency distribution of ultrastructurally distinct FDC subtypes differed between these clusters. In HIV-Clu-2 the frequencies of FDC types with an undifferentiated and regressive morphology occurred at a higher frequency, whereas FDC types with a highly differentiated morphology had a lower frequency. We conclude that 9 out of 15 lymph nodes with HIV-1 associated follicular hyperplasia show changes in FDC morphology indicative of a less differentiated functional stage of FDC. The changes in FDC morphology are closely associated with changes in the germinal center B-cell population resulting in an inverted blast to the centrocyte ratio.

Ultrastructural pathology of human immunodeficiency virus infection

The electron microscope has been used with great skill in many aspects of the acquired immunodeficiency syndrome. It has played a critical role in classifying the human immunodeficiency virus, in characterizing the morphogenesis and gene products of the virus, and in elucidating the host cell targets and interactions. With the aid of the electron microscope, new opportunistic pathogens are being identified, and particularly difficult diagnoses are being made. Extrapolations from observations made at the ultrastructural level to the light microscopic level have provided criteria for the diagnosis of several infectious agents. As with any powerful scientific tool, observations must be interpreted with great care by scientists experienced in electron microscopy.

Inhibition of glutathione synthesis in the liver leads to S-adenosyl-L-methionine synthetase reduction

The hepatic levels of glutathione in rats treated with buthionine sulfoximine (4 mmol/kg), an inhibitor of glutathione synthesis, were 72.5% +/- 4.9% of those determined in control animals. This decrease in glutathione concentration was prevented by the administration of glutathione monoethyl ester (7.5 mmol/kg). S-Adenosyl-L-methionine-synthetase activity in the liver of rats treated with buthionine sulfoximine was 39.4% +/- 6.5% of that determined in control animals. Again, glutathione monoethyl ester prevented the effect of buthionine sulfoximine on S-adenosyl-L-methionine-synthetase activity. There was a close correlation (r = 0.936) between the hepatic levels of glutathione and S-adenosyl-L-methionine-synthetase activity. The hepatic concentration of S-adenosyl-L-methionine in buthionine sulfoximine-treated animals was 59.7% +/- 3.7% of that measured in control rats. Contrasting with the protective effects mentioned above, glutathione monoester had no preventive action on buthionine sulfoximine-induced S-adenosyl-L-methionine depletion. Electron microscopic examination of liver samples of rats after buthionine sulfoximine administration showed evidence of liver degeneration, which was attenuated by glutathione monoethyl ester treatment. Glutathione (7.5 mmol/kg) treatment was less effective than glutathione monoethyl ester in attenuating buthionine sulfoximine effects on hepatic S-adenosyl-L-methionine metabolism and morphology. The reduction of S-adenosyl-L-methionine-synthetase activity observed after treatment with buthionine sulfoximine and its prevention by glutathione monoethyl ester, as well as the correlation between the activity of this enzyme and glutathione levels, indicate that glutathione plays an important role in maintaining S-adenosyl-L-methionine-synthetase activity in the liver.

Isolation of normal human follicular dendritic cells and CD4-independent in vitro infection by human immunodeficiency virus (HIV-1)

Immunohistological and electron microscopy studies of lymph nodes from patients infected with the human immunodeficiency virus 1 (HIV-1) demonstrated that follicular dendritic cells (FDC), the antigen-presenting cells of the B cell system, contain and may produce the virus. To elucidate the mode of infection of FDC with HIV-1 in vitro we developed an improved method for the preparation of single-cell suspensions of viable FDC with high purity (greater than 90% FDC). These isolated FDC were subjected to human T cell leukemia virus IIIB infection, which was monitored after 4 days in culture using the polymerase chain reaction. We were able to demonstrate that normal human FDC are highly susceptible to infection by HIV-1. Inhibition experiments with the monoclonal antibody OKT4a demonstrate that this infection is independent of the CD4 molecule.

Sinus histiocytosis with massive lymphadenopathy (Rosai Dorfman disease) in an HIV-positive patient

We report a case of a 31-year-old HIV infected black female, who presented with asymptomatic generalized lymphadenopathy. Three particularly enlarged lymph nodes were biopsied (2 cervical and 1 axillary). The histological picture was consistent with a diagnosis of sinus histiocytosis with massive lymphadenopathy (SHML) or Rosai-Dorfman disease. Large histiocytes, positive for a variety of macrophage markers and for the S-100 protein, were observed in the distended sinuses. A few hyperplastic follicles, such as usually seen in HIV-infection-associated lymphadenopathy, were present at the periphery of one lymph node. No infections agent besides HIV could be detected by histological or microbiological analysis or by in situ hybridization. This is the first reported case of SHML associated with HIV infection. The possible relationship between the two diseases is discussed.

Monocytoid B-cell lymphoma, a tumour related to the marginal zone

Monocytoid B-lymphocytes are a B-cell subset present in subcapsular sinuses in some cases of lymphadenitis. We describe a case of lymphoma of this cell type. The tumour shows a distinctive morphology characterized by concentric strands of tumour cells around lymphoid follicles with hyperplastic germinal centres and conserved mantle zones. Electron microscopy of these cells shows short cellular processes as well as moderate development of endoplasmic reticulum. The phenotype of the tumour was monoclonal IgM-kappa, distinct from other node-based B-cell subpopulations and suggesting a possible relationship to the lymphocytes of the marginal zone present peripheral to lymphoid follicles of the spleen. Morphological features that suggest a relationship with hairy cell leukaemia are contrasted by phenotypic differences and the ultrastructural absence of ribosomic lamellar complexes.