An AIDS-like condition induced in baboons by HIV-2

Genetically divergent strains of simian immunodeficiency virus use CCR5 as a coreceptor for entry

Entry of human immunodeficiency virus type 1 (HIV-1) requires CD4 and one of a family of related seven-transmembrane-domain coreceptors. Macrophage-tropic HIV-1 isolates are generally specific for CCR5, a receptor for the CC chemokines RANTES, MIP-1alpha, and MIP-1beta, while T-cell line-tropic viruses tend to use CXCR4 (also known as fusin, LESTR, or HUMSTR). Like HIV-1, simian immunodeficiency virus (SIV) requires CD4 on the target cell surface; however, whether it also requires a coreceptor is not known. We report here that several genetically divergent SIV isolates, including SIVmac, SIVsmSL92a, SIVsmLib-1, and SIVcpzGAB, can use human and rhesus CCR5 for entry. CXCR4 did not facilitate entry of any of the simian viruses tested, nor did any of the other known chemokine receptors. Moreover, SIVmac251 that had been extensively passaged in a human transformed T-cell line retained its use of CCR5. Rhesus and human CCR5 differed at only eight amino acid residues, four of which were in regions of the receptor that could be exposed, two in the amino-terminal extracellular region and two in the second extracellular loop. The human coreceptor was as active as the simian for SIV entry. In addition, HIV-1 was able to use the rhesus homologs of the human coreceptors, CCR5 and CXCR4. The SIV strains tested were specific for CCR5 regardless of whether they were able to replicate in transformed T-cell lines or macrophages and whether they were phenotypically syncytium inducing or noninducing in MT-2 cells. However, SIV replication was not restricted to cells expressing CCR5. SIV strains replicated efficiently in the human transformed lymphoid cell line CEMx174, which does not express detectable amounts of transcripts of CCR5. SIV also replicated in human peripheral blood mononuclear cells that were genetically deficient in CCR5. These findings indicated that, in addition to CCR5, SIV can use one or more unknown coreceptors that are expressed on human PBMCs and CEMx174 cells.

Viruses and rickettsiae

In this review I shall try to provide a brief, up-to-date, account of the neuropathology of those viral and rickettsial diseases that are particularly prevalent in tropical regions. These diseases are not, however, exclusive to the tropics. Some, such as AIDS, are common in temperate regions as well, though others are closer to being exclusively tropical, such as some of the arthropod-borne (ARBO) virus encephalides. The latter are dependent for their dissemination on an existence during part of their infectious cycle in insects which are, in turn, climatically and seasonally sensitive. This necessarily limits their geographical distribution. Factors that influence some of the other diseases are less closely dependent on climate and geography and reflect more the social or cultural conditions under which people live. Thus, diseases that depend for their spread on forms of human behavior such as promiscuity or drug abuse (AIDS), or poor hygiene and living conditions (polio, rickettsial diseases) or on contact with domestic and other animals (rabies) may occur in a more widespread distribution, for the tropics are not the only places that afford opportunities for these diseases to flourish. I shall select for discussion aspects of the pathology of these diseases that are currently undergoing investigation but will aim to present these against the backdrop of more established aspects of their pathology. Recent reviews of the pathology of viral encephalitis can be found in Hamilton and Wiley (33) and Esiri and Kennedy (20) and of HIV-1 infection in Price & Sidtis (78) and Scaravilli (85).

The angiogenesis induced by HIV-1 tat protein is mediated by the Flk-1/KDR receptor on vascular endothelial cells

The HIV-1 Tat protein transactivates HIV, viral and some host cell genes. Tat can be released by infected cells and acts extracellularly in the microenvironment, regulating functions of immunocompetent and mesenchymal cells. One of the most striking effects of Tat is the induction of a functional program in vascular cells related to angiogenesis and inflammation (migration, proliferation and expression of plasminogen activator inhibitor-1 and E selectin). Tat induces growth of Kaposi's sarcoma (KS) spindle cells and is angiogenic in vivo and in transgenic mice10-12. We previously reported that Tat is a direct angiogenic factor and noted the Tat arginine- and lysine-rich sequence is similar to that of other potent angiogenic growth factors, such as vascular endothelial growth factor-A (VEGF-A). It is possible that Tat mimics one of these factors by interacting with its growth factor tyrosine kinase receptor. Here we demonstrate that Tat specifically binds and activates the Flk-1/kinase insert domain receptor (Flk-1/KDR), a VEGF-A tyrosine kinase receptor (for review see ref. 13), and that Tat-induced angiogenesis is blocked by agents blocking the Flk-1/KDR receptor. Endothelial cell stimulation by Tat occurs in the absence of activation of FLT-1, another VEGF-A tyrosine kinase receptor.

Viral load and immunophenotype of cells obtained from lymph nodes by fine needle aspiration as compared with peripheral blood cells in HIV-infected patients

We wished to establish the feasibility of fine needle aspiration of lymph nodes as a noninvasive method for measuring subsets of immune cells and viral load in HIV-infected patients. Twenty-five patients (CD4+ T cell range 4-760/microl, median 362) were selected. Lymph node aspiration was attempted in 21 patients. Lymph node cells (LNC), ranging from 6 x 10(3) to 2 x 10(6) (median 6 x 10(5)) were obtained in 17 subjects, and compared with peripheral blood mononuclear cells (PBMC) obtained simultaneously. Immunophenotype could be determined by flow cytometry in 9 patients. Mean percent of CD4+ CD3+ T cells in LNC and PBMC and 23.2 and 14.6. Mean percent of CD8+ CD3+ T cells in LNC and PBMC was 23.1 and 45.0, respectively. Therefore, CD4+/CD8+ ratios were much higher in LNC (mean +/- SD: 1.06 +/- 0.31) than in PBMC (0.35 +/- 0.13). The amount of HIV DNA (11 patients) and RNA (8 patients) was determined in the plasma, LNC, and PBMC by competitive reverse transcriptase-polymerase chain reaction (RT-PCR). The number of copies of viral DNA/10(5) cells was higher in LNC than in PBMC (LNC/PBMC ratio ranger: 0.54-25, median 3.4). The number of copies of unspliced viral RNA/10(5) cells was much higher in LNC than in PBMC (LNC/PBMC ratio range 65-1,159, median 435). The plasma RNA copy number, a measure of circulating cell-free virus, was correlated with the RNA copy number in PBMC, but not in LNC. A RT-PCR system specific for spliced transcripts was also used to assess the level of transcripts independent of genomic RNA. This assay also detected more signal in LNC than in PBMC. The level of spliced transcripts in LNC and PBMC correlated with the amount of full-length RNA detected by competitive PCR. A semiquantitative coculture assay with lymphoblasts from healthy donors was used to assess the infectivity of LNC as compared with PBMC in 14 patients. The minimum number of LNC necessary to cause a positive coculture ranged from 10(3) to > 10(5) (median 10(4)); the corresponding number for PBMC ranged from 10(3) to > 10(6) (median 5 x 10(5)). In most patients selected for palpable lymph nodes, LNC could be obtained by fine needle aspiration, thus allowing noninvasive monitoring of viral burden in lymphoid tissue. The present study also suggests that both T-cell subsets and viral load differ in the blood and lymphoid tissue, which raises the question of whether the study of the lymphoid tissue would yield better prognostic markers of disease course.

The value of primate models for studying human immunodeficiency virus pathogenesis

Research on human immunodeficiency virus (HIV) infection is compromised by the obvious limitation in having for study only virus-infected individuals or those exposed to the virus. Steps involved in transmission or pathogenesis require planned experimentation. The identification of animal models of acquired immunodeficiency syndrome (AIDS) has therefore been helpful for evaluating phases of HIV pathogenesis. Of the seven subgenera of lentiviruses now recognized, two share the characteristics with HIV of a T cell tropism and the associated loss of CD4+ cells in the host associated with disease: the feline immunodeficiency virus (FIV) and the simian immunodeficiency virus (SIV) (Table 1). The other animal lentiviruses grow best in macrophages and their infection generally reflects clinical sequellae of infection of this cell type. This review addresses those features of SIV, HIV, and SHIV infections of non-human primates that illustrate the importance of the animal models of AIDS.

Chimeric simian/human immunodeficiency virus that causes progressive loss of CD4+ T cells and AIDS in pig-tailed macaques

By animal-to-animal passage of simian/human immunodeficiency virus (SHIV) in pig-tailed macaques, we have developed a macaque model of human immunodeficiency virus type 1 (HIV-1) disease in humans. Passaging was begun with a chimeric virus containing the env gene of HIV-1 HXBc2 and the gag and pol genes of simian immunodeficiency virus SIVmac239. SHIV was passaged serially in cohorts of two macaques each, using bone marrow-to-bone marrow transfers at 5, 5, and 16 weeks for passages 2, 3, and 4, respectively. The fifth passage was done by using cell-free virus isolated from cerebrospinal fluid of a passage 4 macaque. The virus became more virulent with each passage. Virus replication was restricted in all three animals in passages 1 and 2 but not in five of the six animals in passages 3, 4, and 5. In these animals, intense virus replication in the lymphoid tissues resulted in almost total elimination of CD4+ T cells within weeks of inoculation, and three of these animals developed AIDS in less than 1 year. The more uniform virus-host interaction initiated by the cell-free virus in the passage 5 animals contrasted with a more variable pattern of disease initiated by infectious bone marrow cells during earlier passages. The virulent cell-free SHIV can now be used to screen the efficacy of vaccines directed against the envelope of HIV-1.

Developing animal models for AIDS research--progress and problems

Tremendous advances in our understanding of the human immunodeficiency viruses (HIVs) have been made through the use of animal models. However, there are limitations inherent in many of the current models that use either nonhuman primates, or viruses other than HIV-1. Researchers continue to search for improved models using small-animal model alternatives and different viruses. Animal models will remain an important tool in our continued search for vaccines and therapeutic agents.