Non-Myeloid Cell Phagocytosis

*Ben A. Calvert and Amy L. Ryan*

### **Abstract**

As professional phagocytes, myeloid cells, including macrophages, dendritic cells, and neutrophils, are often the targets for investigation and analysis of phagocytosis. Phagocytosis, however, has also been observed in nonmyeloid cells, including epithelium, mesenchymal, and smooth muscle cells. Colloquially known as nonprofessional phagocytes, these nonmyeloid cells are capable of phagocytosis of pathogenic material and efferocytosis of apoptotic bodies. Cells, such as those found in the epithelium, are often the primary site for viral and bacterial infection and have evolved to possess strong anti-pathogenic machinery of their own. The processes by which nonmyeloid cells can engage in phagocytic functions have wide implications for tissue homeostasis and disease pathogenesis, including infection and colonization. This chapter will review the phagocytosis capabilities in these nonmyeloid cells.

**Keywords:** efferocytosis, epithelial cells, internalization, barrier, nonprofessional, opsonization, trigger phagocytosis, zipper phagocytosis

### **1. Introduction**

As professional phagocytes, myeloid cells, including neutrophils, macrophages, monocytes, mast cells, and dendritic cells, are actively recruited to sites of tissue damage, infection, and inflammation playing a key role in host defense [1]. Of these, neutrophils and macrophages are perhaps the most widely studied in terms of their roles in phagocytosis [2–4]. However, there is increasing evidence that nonmyeloid cells, including epithelial [5, 6] endothelial [7–9], mesenchymal [7, 10–12], and smooth muscle cells [13–16], can also engage in phagocytosis, or phagocytic-like mechanisms when phagocytosis is not their principal function. Phagocytosis by nonprofessional phagocytes is often referred to as internalization or even cannibalism, especially in the case of efferocytosis of apoptotic neighboring cells [17]. Nonprofessional phagocytes were first distinguished from professional phagocytes as early as 1970 after Rabinovitch demonstrated particulate uptake in fibroblasts [18, 19], although reports had demonstrated particulate uptake in nonmyeloid cells almost 40 years prior [20]. Since this initial observation, many nonprofessional phagocytes have been identified to have the phagocytic capacity and the capacity to clear potentially dangerous pathogens [21]. **Table 1** includes a summary of these cell types and the roles that they have been observed to play in phagocytosis. Compared to professional phagocytes, nonmyeloid cells engage in distinctively different


*Phagocytosis – Main Key of Immune System*


## *Non-Myeloid Cell Phagocytosis DOI: http://dx.doi.org/10.5772/intechopen.110583*


**Table 1.**

*Mesenchymal Stem Cells.*

*Key studies in nonmyeloid cell phagocytosis.*

#### *Non-Myeloid Cell Phagocytosis DOI: http://dx.doi.org/10.5772/intechopen.110583*

mechanisms to recognize, engulf, and destroy pathogens through phagocytosis. Nonprofessional phagocytes are demonstrably less efficient and lack factors such as Pattern Recognition Receptors (PRRs) capable of recognizing Pathogen Associated Molecular Patterns (PAMPs), as well as reactive oxygen species (ROS) and degradation enzymes required for effective clearance and degradation [19]. Nonmyeloid cells, however, provide a significant contribution toward the clearance of exogenous pathogens, cellular debris, and apoptotic bodies *via* phagocytosis, and what they lack in efficiency, can make up for in cell number [5, 57]. This chapter will focus on the specific functions of nonprofessional phagocytes, highlighting their differences from professional phagocytes and their specific and important contribution to tissue homeostasis.
