HTLV-1-infected cells drive the differentiation of monocytes into macrophages in vitro.

Background: The human T-cell lymphotropic virus type 1 (HTLV-1) is a retrovirus that causes HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HAM/TSP is a chronic inflammatory neurodegenerative disease characterized by leukocyte infiltration in the spinal cord. T-lymphocytes are the most important targets of HTLV-1 infection, but monocytes are also infected. Monocytes from HTLV-1-infected individuals exhibit important functional differences compared to cells from uninfected donors. Here, we investigated the effects of cell-cell physical contact and/or secreted factors of HTLV-1-infected cells in monocyte activation and differentiation.

Methods: The THP-1 human monocytic cell line was co-cultured with a human cell line transformed by HTLV-1 (MT-2) for 6 days. To determine the effects of co-culturing HTLV-1-infected cells in THP-1 monocytes cells were characterized by flow cytometry, immunofluorescence microscopy, and real-time PCR. Computational analysis of published transcriptomic datasets was realized to compare molecular profiles of macrophages and mononuclear cells from HTLV-1 carriers.

Results: Co-culture of monocytes with HTLV-1-infected cells induced macrophage differentiation and upregulation of typical macrophages-associated molecules (HLA-DR, CD80, and CD86), increased cytokine (TNFα, IL-6, and IL-1β) levels and their coding genes expression. Consistently, published transcriptomic datasets showed changes in important genes associated with inflammation during HAM/TSP in patients. The presence of HTLV-1-infected cells in the culture also induced significant upregulation of Interferon Stimulated Genes (ISG), indicating viral infection. Monocyte activation and differentiation into pro-inflammatory macrophages occurred in a cell-to-cell contact-independent manner, suggesting the role of factors secreted by infected cells.

Conclusions: Together, our results indicated that HTLV-1-infected cells induced monocyte differentiation into macrophages inflammatory, predominantly.