Characterization of a Human Stromal Cell Line Supporting Hematopoietic Progenitor Cell Proliferation: Effect of HIV Expression

Suppression of clonogenic potential of human bone marrow mesenchymal stem cells by HIV type 1: putative role of HIV type 1 tat protein and inflammatory cytokines

Bone marrow abnormalities are frequently observed in HIV-1-infected individuals. Infection of marrow mesenchymal stem cells (MSCs) may abrogate their growth properties and hematopoietic supportive functions. To delineate the cell type infected, and factors responsible for the deleterious effects, human bone marrow cells were exposed to HIV-1 in vitro. By week 4, the ability of MSCs to form colonies of purely fibroblasts (CFU-F) and mixed colonies of fibroblasts and adipocytes (CFU-FA) was suppressed by 23 +/- 5 and 55 +/- 7%, respectively. The p24 concentration in culture supernatants steadily declined from 170 ng/ml in the inoculum to 134 +/- 30, 35 +/- 15, 2.3 +/- 3, and <0.02 ng/ml at the end of week 1, 2, 3, and 4, respectively. However, even at week 4, coculturing with MT-4 lymphocytes for 1 week dramatically increased p24 levels. Polymerase chain reaction (PCR) amplification, using HIV-1-specific primers, and in situ hybridization with an HIV-1 cDNA probe demonstrated the presence of virus-specific nucleic acids within stromal colonies. Coimmunostaining with antibody to CD83 implicated the presence of HIV-1 within dendritic progenitor cells. Immunostaining with HIV-1 Tat antibody demonstrated the presence of Tat protein and reverse transcriptase (RT)-PCR assays showed increased (160-220%) mRNA levels for inflammatory cytokines (tumor necrosis factor alpha [TNF-alpha], interleukin 1beta [IL-1beta], IL-6, and macrophage inflammatory protein 1alpha [MIP-1alpha]). A concentration-dependent decrease in CFU-STROs was observed on incubation with either Tat protein (1-100 ng/ml) or with TNF-alpha or IL-1beta (0.025-25 ng/ml). These results suggest that HIV-1 infection of stromal cells may produce inhibitory factors that suppress the clonogenic potential of MSCs.

Pathogenesis of HIV-1 infection within bone marrow cells

Mononuclear phagocytic cells and CD4+ T lymphocytes represent the major targets for infection by HIV-1 in vivo. The most severe pathogenic features associated with HIV-1 infection can be attributed to malfunction or premature death of these cells that are of hematopoietic origin. Patients with acquired immunodeficiency syndrome (AIDS), suffer from many hematologic disorders, particularly those persons with long-term infection of HIV-1. These disorders include anemia, lymphocytopenia, thrombocytopenia and neutropenia. The mechanisms that lead to the induction of these disorders are multi-factorial. However, sufficient evidence has accumulated which suggests that HIV-1 infection of cells within the microenvironment of the bone marrow can lead to the induction of hematopoietic deficits. Most studies indicate that marrow-derived hematopoietic stem cells cannot be infected by HIV-1 until they undergo modest differentiation in order to express the appropriate receptors to enable virus entry and subsequent replication. Some cells within the mixed environment of the marrow stroma appear to support HIV-1 replication however. These cells include marrow microvascular endothelial cells, sometimes referred to as blanket cells, stromal fibroblasts, as well as mononuclear phagocytes. Our recent experiments suggest that the HIV-1 accessory protein, Vpr, plays some role in the activation of marrow-derived mononuclear phagocytes which appears to result in premature phagocytosis of non-adherent marrow cells present in the in vitro cultures. This phenomenon could account, in part, for the induction of cytopenias that are typical of individuals infected by HIV-1.

Providing a Microenvironment for the Development of Human CD34+ Hematopoietic Cells in SCID Mice

In order to develop a convenient small-animal model that can support the differentiation of human bone-marrow-derived CD34+ cells, we transplanted SCID mice with an immortalized human stromal cell line, Lof(11-10). The Lof(11-10) cell line has been characterized to produce human cytokines capable of supporting primitive human hematopoietic cell proliferation in vitro. Intraperitoneal injection of Lof(11-10) cells into irradiated SCID mice by itself resulted in a dose-dependent survival of the mice from lethal irradiation. The radioprotective survival was reflected by an increase in the growth and number of mouse bone-marrow-derived committed hematopoietic progenitors. The Lof(11-10) cells localized to the spleen, but not to the bone marrow of these animals and resulted in detectable levels of circulating human IL-6 in their plasma. Secondary intravenous injections of either human or simian CD34+ cells into the Lof(11-10)-transplanted SCID mice resulted in engraftment of injected cells within the bone marrow of these mice. The utility of this small-animal model that allows the growth and differentiation of human CD34+ cells and its potential use in clinical gene therapy protocols are discussed. Copyright 1997 S. Karger AG, Basel