RESTRICTED HIV TYPE-1 REPLICATION UNDER SERUM-FREE CULTURE CONDITIONSIN HUMAN MONOCYTE-DERIVED MACROPHAGES

Monocytes (MOs) and macrophages (MACs) are well-known targets for HIV- 1 infection. Even though the virus load is contributed mainly to lymph ocytes during the asymptomatic phase of infection, the expression of H IV-1 in MO/MACs seems to be important for the course of the disease. T o establish a model for restricted HIV-1 expression in MACs in vitro, we cultured MO-derived MACs under different culture conditions and ana lyzed their susceptibility to HIV-1 infection as well as their capacit y for virus replication in vitro. MACs cultured under serum-free condi tions with M-CSF (M-MACs) remain viable and functionally active as ass essed by the analysis of cytokine production. In addition, the levels of CD4, CD14, CCR5, and HLA-DR expression are comparable to those of s erum-derived MACs (SER-MACs). However, serum-free MACs were less susce ptible to HIV-1 infection, with only 9.5 +/- 4.5% (mean +/- SEM) of al l cells being p24 antigen positive on day 22 as compared with 51 +/- 9 % under serum conditions (p < 0.005). Reverse transcriptase (RT) activ ity in the culture supernatant of M-MACs was always about 100-fold low er than that of SER-MACs even when comparable amounts of cells were in fected. The addition of serum to serum-free cultures increased the per centage of HIV-1 p24 antigen-positive cells (21 +/- 8% positive cells on day 22) and increased the RT activity, indicating that serum factor s could be important for HIV-1 replication in MACs. Therefore we also switched SER-MACs to serum-free culture conditions and found a sharp d ecrease in RT activity. However, the RT level could always be rescued by the addition of serum, even after a long serum-free culture period. This effect was dependent on the serum concentration added, with as l ittle as 0.1% serum being effective in reestablishing viral production as measured by RT activity. In conclusion, we show that serum has an important role in the replication of HIV-1 in MACs. Our results sugges t that besides the role of CD4 and CCR5 other microenvironmental facto rs, e.g., growth factors, cytokines, or hormones, which are not provid ed by the target cell itself, are involved in the regulation of MAC in fection and of replication by HIV-1.