The Biology of Monocytes and Dendritic Cells: Contribution to HIV Pathogenesis

Abstract

Myeloid cells such as monocytes, dendritic cells (DC) and macrophages (MΦ) are key components of the innate immune system contributing to the maintenance of tissue homeostasis and the development/resolution of immune responses to pathogens. Monocytes and DC, circulating in the blood or infiltrating various lymphoid and non-lymphoid tissues, are derived from distinct bone marrow precursors and are typically short lived. Conversely, recent studies revealed that subsets of tissue resident MΦ are long-lived as they originate from embryonic/fetal precursors that have the ability to self-renew during the life of an individual. Pathogens such as the human immunodeficiency virus type 1 (HIV-1) highjack the functions of myeloid cells for viral replication (e.g., MΦ) or distal dissemination and cell-to-cell transmission (e.g., DC). Although the long-term persistence of HIV reservoirs in CD4+ T-cells during viral suppressive antiretroviral therapy (ART) is well documented, the ability of myeloid cells to harbor replication competent viral reservoirs is still a matter of debate. This review summarizes the current knowledge on the biology of monocytes and DC during homeostasis and in the context of HIV-1 infection and highlights the importance of future studies on long-lived resident MΦ to HIV persistence in ART-treated patients.

Keywords: CD16; HIV; antiretroviral therapy; dendritic cells; macrophages; monocytes.

Figure 1

Human monocytes are classified into three subtypes based on the differential expression of CD14 and CD16: classical CD14++CD16− (also referred as CD14+ or CD14+CD16−), intermediate CD14++CD16+ (also referred as CD14+CD16+ or CD14+CD16int/low) and non-classical CD14+CD16++ (also referred as CD14LowCD16+ or CD14dimCD16+) monocytes.

Figure 2

Human DC are divided into three major subsets: plasmacytoid DC (pDC), myeloid DC (mDC) and monocyte-derived DC (MDDC).