CD169+ Macrophages Capture and Dendritic Cells Instruct: The Interplay of the Gatekeeper and the General of the Immune System

Abstract

Since the seminal discovery of dendritic cells (DCs) by Steinman and Cohn in 1973, there has been an ongoing debate to what extent macrophages and DCs are related and perform different functions. The current view is that macrophages and DCs originate from different lineages and that only DCs have the capacity to initiate adaptive immunity. Nevertheless, as we will discuss in this review, lymphoid tissue resident CD169⁺ macrophages have been shown to act in concert with DCs to promote or suppress adaptive immune responses for pathogens and self-antigens, respectively. Accordingly, we propose a functional alliance between CD169⁺ macrophages and DCs in which a division of tasks is established. CD169⁺ macrophages are responsible for the capture of pathogens and are frequently the first cell type infected and thereby provide a confined source of antigen. Subsequently, cross-presenting DCs interact with these antigen-containing CD169⁺ macrophages, pick up antigens and activate T cells. The cross-priming of T cells by DCs is enhanced by the localized production of type I interferons (IFN-I) derived from CD169⁺ macrophages and plasmacytoid DCs (pDCs) that induces DC maturation. The interaction between CD169⁺ macrophages and DCs appears not only to be essential for immune responses against pathogens, but also plays a role in the induction of self-tolerance and immune responses against cancer. In this review we will discuss the studies that demonstrate the collaboration between CD169⁺ macrophages and DCs in adaptive immunity.

Keywords: CD169; T cell; antigen; cross-presentation; dendritic cells; macrophages; sialoadhesin; siglec-1.

Figure 1

The different functions of CD169⁺ macrophages and their cross-talk with cDC1. (1) Uptake: CD169⁺ macrophages capture and phagocytose pathogens, including bacteria and viruses, as well as dead cells. The CD169 molecule also directly binds to exosomes and specific pathogens, such as HIV. (2) Antigen transfer: CD169⁺ macrophages directly interact and present antigens to cDC1s for the generation of antigen-specific CD8⁺ T cell responses. While HIV particles are transferred via CD169, other components of bacteria and viruses can be transferred to cDC1s from the macrophages. Dead cells can stimulate cDC1s via CLEC9A expressed on the cDC1. The interaction between CD169⁺ macrophages and cDC1s is dependent on binding of CD169 to sialic acid structures on cDC1s. (3) IFN-I priming: after encounter with bacteria, dead cells, or viruses, CD169⁺ macrophages secrete IFN-I that is required for optimal activation of cDC1s and T cells. Subsequently, pDCs are recruited and their IFN-I production further amplifies the signal. (4) Trans-infection: in the case of HIV and MLV, CD169⁺ macrophages can also mediate viral trans-infection to CD4 T cells and B cells.