Natural killer (NK) cells are specialized white blood cells of the innate immune system, acting as a rapid response team against various threats. They patrol for abnormal cells, such as those infected by viruses or cancerous cells. NK cells are known for directly eliminating such compromised cells. This raises a question: do these cells also engulf and digest foreign particles, a process known as phagocytosis?
What Are Natural Killer Cells?
Natural killer cells originate in the bone marrow and act without prior exposure to specific pathogens. As part of the innate immune system, they provide immediate defense. Their primary function involves recognizing and destroying infected or cancerous cells.
NK cells identify target cells by recognizing a lack of specific surface markers, often called “missing self” markers. Upon identification, NK cells release cytotoxic granules. These granules contain proteins like perforin, which creates pores in the target cell’s membrane, and granzymes, which then trigger programmed cell death. This mechanism allows NK cells to quickly eliminate threats without causing widespread damage to healthy tissues.
Understanding Phagocytosis
Phagocytosis is a fundamental biological process where certain cells engulf and digest particles, such as microbes, dead cells, or cellular debris. It is a key immune process, defending against pathogens and clearing cellular waste to maintain tissue health. This regulated process involves recognizing target particles, engulfing them into a membrane-bound vesicle called a phagosome, and subsequent degradation.
Professional phagocytes are the primary cells responsible for this process. These include macrophages, large, long-lived tissue-resident cells involved in immediate defense and antigen presentation. Neutrophils are another professional phagocyte, known for rapid arrival at infection sites and efficient engulfment and killing of bacteria. Dendritic cells also exhibit phagocytic activity, particularly for processing and presenting antigens to activate adaptive immune responses.
When Natural Killer Cells Exhibit Phagocytic Activity
While not typically categorized as professional phagocytes, emerging research indicates that human NK cells can exhibit phagocytic activity under specific conditions. This activity is not as widespread or robust as that of dedicated phagocytes like macrophages or neutrophils. However, it represents a more complex and versatile role for NK cells than previously understood.
One example is their ability to phagocytose certain fungal pathogens, such as Candida albicans. This interaction with C. albicans has been shown to trigger NK cell activation, leading to degranulation and the secretion of pro-inflammatory cytokines like granulocyte-macrophage colony-stimulating factor, interferon gamma, and tumor necrosis factor alpha. Activation of NK cells by C. albicans has been observed to be dependent on actin-mediated phagocytosis, linking engulfment and their immune response.
This phagocytic capability can also modulate the activity of other immune cells. For instance, NK cells enhance the antifungal activity of polymorphonuclear neutrophils (PMNs) against C. albicans. Although NK cells can exhibit fungicidal activity through secreted perforin, they generally do not inhibit the filamentation of C. albicans. This suggests their phagocytic role, while not primary, contributes to the overall immune response by activating themselves and influencing other immune players, highlighting a nuanced interaction in host defense.