Productive human immunodeficiency virus-1 infection of purified megakaryocytic progenitors/precursors and maturing megakaryocytes.

We have evaluated the susceptibility to human immunodeficiency virus (HIV)-1 infection of in vitro grown megakaryopoietic progenitors/precursors and maturing megakaryocytes (MKs), based on the following approach: (1) human hematopoietic progenitor cells (HPCs), stringently purified from peripheral blood and grown in serum-free liquid suspension culture supplemented with thrombopoietin (Tpo), generated a relatively large number of >/= 98% to 99% pure megakaryocytic precursors and then mature-terminal MKs; (2) at different days of culture (ie, 0, 5, 8, 10) the cells were inoculated with 0.1 to 1.0 multiplicity of infection (m.o.i.) of the lymphotropic NL4-3 or 0.1 m.o. i. of the monocytotropic BaL-1 HIV-1 strain; (3) finally, the presence of viral mRNA and proteins was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR)/in situ hybridization and antigen capture assays, respectively, on day 2 to 12 of culture. MKs derived from day 0 and day 5 BaL-1-challenged cells do not support viral replication as assessed by p24 enzyme-linked immunosorbent assay (ELISA) and RT-PCR. On the contrary, HIV transcripts and proteins were clearly detected in all NL4-3 infection experiments by RT-PCR and p24 assay, respectively, with the highest viral expression in day 5 to 8 challenged MKs. In situ hibridization studies indicate that the percentage of HIV+ MKs varies from at least 1% and 5% for day 0 and day 5 infected cells, respectively. Production of an infectious viral progeny, evaluated by the capability of culture supernatants from day 5 NL4-3-challenged MKs to infect C8166 T-lymphoblastoid cell line, was consistently observed (viral titer, approximately 5 x 10(3) tissue culture infectious dose50/mL/10(6) cells). Exposure of MKs to saturating concentration of anti-CD4 OKT4A monoclonal antibody (MoAb), which recognizes the CD4 region binding with the gp120 envelope glycoprotein, markedly inhibited HIV infection, as indicated by a reduction of p24 content in the supernatants: because the inhibitory effect was incomplete, it is apparent that the infection is only partially CD4-dependent, suggesting that an alternative mechanism of viral entry may exist. Morphologic analysis of day 12 MKs derived from HPCs infected at day 0 showed an impaired megakaryocytic differentiation/maturation: the percentage of mature MKs was markedly reduced, in that approximately 80% of cells showed only one nuclear lobe and a pale cytoplasm with few granules. Conversely, megakaryocytic precursors challenged at day 5 to 8 generated fully mature day 10 to 12 MKs showing multiple nuclear segmentation. Thus, the inhibitory effect of HIV on the megakaryopoietic gene program relates to the differentiation stage of cells subjected to the viral challenge. Finally, HPCs treated with 20 or 200 ng/mL of recombinant Tat protein, analyzed at different days of culture, showed an impaired megakaryocytopoiesis comparable to that observed in HIV-infected cells, thus suggesting that Tat is a major mediator in the above described phenomena. These results shed light on the pathogenesis of HIV-related thrombocytopenia; furthermore, they provide a model to investigate the effects of HIV on megakaryocytic differentiation and function.