Pathogenicity of HIV in lymphatic organs of patients with AIDS

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.

Hyperplastic lymphoid tissue in HIV/AIDS: an electron microscopic study

The purpose of this study was to characterize the ultrastructure of lymphoid tissue from HIV/AIDS patients and to evaluate it as a reservoir and source of HIV. HIV has been demonstrated in lymph nodes and tonsils and adenoids, by immunohistochemistry (IHC), in situ hybridization (ISH), and transmission electron microscopy (TEM), to be associated with germinal center (GC) follicular dendritic cells (FDC). The presence of HIV in the larger gastrointestinal tract-associated lymphoid tissue (GALT) has been much less studied. Whether FDC themselves are productively infected by HIV in any of the lymphoid sites is controversial. Lymph nodes, tonsils, and gastrointestinal biopsies were fixed in neutral buffered glutaraldehyde and prepared for TEM. Mature HIV particles were abundant in GC of hyperplastic lymph nodes, tonsils, and the GALT. They were enmeshed within an electron-dense matrix associated with an all-encompassing branching FDC network of processes. HIV particles were seen budding from both FDC and lymphocytes. The greatest numbers of particles were seen in hyperplastic lymphoid tissue from untreated individuals and in lymph nodes co-infected with opportunistic organisms, such as Mycobacterium avium complex. In addition to HIV, unidentifiable "particles" of varying sizes, possibly including other viruses, were regularly seen in association with FDC. Ultrastructural study graphically demonstrated the abundance of HIV particles associated with the complex FDC network of hyperplastic lymph nodes, tonsils, and GALT. HIV was shown to productively infect FDC, as well as lymphocytes.

Characterization of a thymus-tropic HIV-1 isolate from a rapid progressor: role of the envelope

Loss of T cell homeostasis usually precedes the onset of AIDS. We hypothesized that rapid progressors may be transmitted with HIV-1 that is particularly able to perturb T cell homeostasis. To this end, we have tested two transmitted, syncytium-inducing (SI) viral isolates from a rapid progressor in two thymus models. One of the isolates (R3A) exhibited markedly rapid kinetics of replication and thymocyte depletion. These phenotypes mapped to the envelope, as a recombinant NL4-3 virus encoding the R3A envelope had similar phenotypes, even in the absence of nef. Notably, the viruses with high pathogenic activity in the thymus (R3A and NL4-R3A) did not show enhanced replication or cytopathicity in PHA-stimulated PBMCs. Furthermore, NL4-R3A did not enhance replication of the coinfected NL4-3 virus in the thymus, suggesting an intrinsic advantage of the R3A envelope. The R3A envelope showed higher entry activity in infecting human T cells and in depleting CD4+ thymocytes when expressed in trans. These data suggest that SI viruses with unique envelope functions which can overcome barriers to transmission may hasten disease progression by perturbing T cell homeostasis.

HIV-1 replication and pathogenesis in the human thymus

How HIV replicates and causes destruction of the thymus, and how to restore thymic function, are among the most important questions of HIV-1 pathogenesis and therapy in adult as well as pediatric patients. The thymus appears to function, albeit at reduced levels, throughout the life of adults, to respond to T cell depletion induced by HIV and to be suppressed by HIV. In this review, we summarize recent findings concerning HIV replication and pathogenesis in the human thymus, focusing on mechanistic insights gleaned from studies in the SCID-hu Thy/Liv mouse and human fetal-thymus organ culture (HF-TOC) models. First, we discuss HIV viral determinants and host factors involved in the replication of HIV in the thymus. Second, we consider evidence that both viral factors and host factors contribute to HIV-induced thymocyte depletion. We thus propose that multiple mechanisms, including depletion and suppression of progenitor cells, paracrine and direct lytic depletion of thymocytes, and altered thymocyte selection are involved in HIV-induced pathology in the thymus. With the SCID-hu Thy/Liv mouse and HF-TOC models, it will be important in the coming years to further clarify the virological, cell biological, and immunological mechanisms of HIV replication and pathogenesis in human thymus, and to correlate their significance in HIV disease progression.

The Warthin-Finkeldey-type giant cell in HIV infection, what is it?

A study was conducted to ascertain the origin of the Warthin-Finkeldey-type giant cell that is common to lymphoid tissues of HIV-infected individuals. Light microscopy (LM) and transmission electron microscopy (TEM), in situ hybridization (ISH) (HIV-specific RNA), and immunohistochemistry (HIV p24, OPD4, CD3, CD45 UCHL, CD21, CD35, S-100, p55 (actin-bundling protein), CD68, HAM56, alpha-1-antitrypsin, alpha-1-antichymotrypsin, and lysozyme) studies were performed on hyperplastic tonsil, adenoid, lymph node, and intestinal MALT specimens from HIV+ patients. Warthin-Finkeldey-type giant cells (WFTGC) and follicular dendritic cells (FDC) shared characteristic morphologic (high N: C ratio; crowded, irregular nuclei; thin filaments with dense bodies; desmosomes; and cilia) and immunophenotypic (CD21+, CD35+, S-100+, p55, and vimentin+) features. Also, transitional forms between binucleated FDC and WFTGC were identified by TEM. TEM and ISH revealed evidence of HIV expression by FDC, but not WFTGC. WFTGC in HIV- lymphoid specimens displayed identical LM and IHC characteristics. The WFTGC in HIV infection appears to represent a multinucleated form of FDC.

Thymic pathology in primary and secondary immunodeficiencies

For the sake of clarity and in agreement with the World Health Organization immunodeficiency classification, it is important to distinguish the congenital, inherited malformative lesions called generically 'thymic dysplasia' from the secondary, acquired changes, designated under the broad term of 'severe thymic atrophy'. Thymic dysplasia represents the archetype of thymic changes in cellular immunodeficiency, since there is no example of a thymic dysplasia associated with a normal T-cell function. Thymic dysplasia is observed in several inherited diseases, the most frequent of them being severe combined immunodeficiency. More than the depletion of lymphoid cells, the lack of differentiation of the thymic epithelium, responsible for the absence of Hassal's corpuscles, is the main and constant feature of this condition. Thymic dysplasia underscores the crucial role of the thymic epithelium in the normal differentiation of the T-cell population. Severe thymic atrophy is secondary to various causes, including prolonged protein malnutrition and immunosuppressive or cytotoxic drugs, graft versus host reaction and, chiefly today, chronic viral infection, especially with HIV-1. The morphological changes are similar and are characterized by a partial lymphoid depletion, involving mainly the CD1+ population, necrosis and calcification of epithelial cells, the frequent presence of plasma cells and, more significantly, fibrohyaline changes of the basement membrane of the vessels and thymic epithelium. The severity of the atrophic changes and the immunodeficiency-related manifestations depend on the duration of the aetiological factors and, more significantly, with their early occurrence, within the first months of life. The mechanisms underlying thymic atrophy are poorly understood. A primary impairment of lymphoid cells seems at present to be the most likely hypothesis.

Production of interleukins in human immunodeficiency virus-1-replicating lymph nodes

To document the in vivo interactions occurring between the immune system and HIV replicating cells, we analyzed using in situ hybridization the production of IL-1 beta, IL-6, IL-2, and INF-gamma in eight hyperplastic lymph nodes from HIV-1 infected patients. Numerous IL-1 beta- and IL-6-producing cells associated in clusters were detected in sinuses. Few individual IL-1 beta- and IL-6-producing cells were present in interfollicular and follicular areas. IL-2- and INF-gamma-producing cells were observed in all lymph node compartments, with a selective enrichment in germinal centers. The amount and distribution of IL-1 beta, IL-6-, and IL-2-producing cells in HIV lymph nodes were not different from those found in six HIV unrelated hyperplastic lymph nodes. In contrast, a higher level of INF-gamma production was observed in HIV-1 lymph nodes. The CD8+ cells that accumulate in germinal centers of HIV lymph nodes (and not in non-HIV germinal centers) were actively involved in this INF-gamma production. INF-gamma synthesizing cells were in direct contact with cells containing HIV core antigens and HIV RNA. Thus a high INF-gamma production may characterize anti-HIV T cell immune response, potentially contributing to control of viral spreading as well as to the development of follicle lysis.

Detection by in situ hybridization of HIV I RNA in spleens of HIV I sero-positive patients with thrombocytopenic purpura

In situ hybridization with a 3H labelled probe on cryosections from 6 spleens of HIV I sero-positive patients with thrombocytopenic purpura showed the presence of HIV RNA in 4 of the 6 spleens at the follicular hyperplasia stage. Two patterns of hybridization were observed: first, a diffuse autoradiographic signal, displaying an irregular network, detected in 1 or 2 germinal centres (GC) per section (17%); secondly, the presence of very few distinct radioactive cells in the labelled GC. A similar pattern was observed in an ARC (Acquired immunodeficiency syndrome-Related Complex) lymph node, but with a more intense and frequent hybridization signal. These results indicate that the spleens, like the lymph nodes, are involved in the course of HIV infection but with a less intense tissue-virus interaction, which may explain the minor morphological changes observed in the spleens. In addition, a careful examination of the lymph node tissue indicated that lymphocytes are the predominant cell type infected with the virus. As for the follicular dendritic cells (FDC), a similarity of the hybridization signal observed in the GC and in vitro HIV infected cells suggests that the FDC could also be sensitive to the virus.

Antigen detection for human immunodeficiency virus

The recent development of enzyme immunoassay procedures for the direct determination of human immunodeficiency virus (HIV) antigens has been of significant benefit in both clinical and research applications. The historical development of HIV antigen assays as well as their current and future applications for use in the clinical microbiology laboratory are reviewed. A detailed description of selected commercially available assays is presented, and a comparison is made of various parameters, including sensitivity, specificity, and cost. The use of the HIV antigen assay as an alternative to the reverse transcriptase assay in virus culture applications is also discussed. In addition, the diagnostic and prognostic utility of the HIV antigen assay is considered for various patient groups, including neonatal, high-risk asymptomatic, seronegative, and seropositive patient populations. The use of the HIV antigen assay as an adjunct to anti-HIV antibody testing, as well as its utility in assessing the therapeutic efficacy of antiviral drug therapy, is discussed. The biology of HIV antigen expression and modulation of anti-HIV antibody titers during infection are also discussed in terms of two possible models.

Lymph node enlargement in patients with unsuspected human immunodeficiency virus infections

The histologic findings in lymph nodes were used to identify eight patients, who are not in a high-risk group, with human immunodeficiency virus (HIV) infection. In order to determine the specificity of these findings, the histologic and clinical findings in these patients were compared with the histologic and clinical findings in 40 patients whose lymph nodes exhibited reactive follicular hyperplasia and who received biopsies before 1981. While a definitive diagnosis of HIV infection cannot be made from the histologic changes in lymph nodes because the organisms cannot be identified, our findings indicate that HIV infection can be suggested, and appropriate testing warranted, when marked reactive follicular hyperplasia with mononuclear cells (and a small number of neutrophils) in parafollicular sinuses is found in a patient with unexplained lymph node enlargement at two or more noncontiguous, noninguinal sites for several months, with or without systemic symptoms.