Natural Killer (NK) Cells in Antibacterial Innate Immunity: Angels or Devils?

Abstract

Natural killer (NK) cells were first described as immune leukocytes that could kill tumor cells and soon after were reported to kill virus-infected cells. In the mid-1980s, 10 years after their discovery, NK cells were also demonstrated to contribute to the fight against bacterial infection, particularly because of crosstalk with other leukocytes. A wide variety of immune cells are now recognized to interact with NK cells through the production of cytokines such as interleukin (IL)-2, IL-12, IL-15 and IL-18, which boost NK cell activities. The recent demonstration that NK cells express pattern recognition receptors, namely Toll-like and nucleotide oligomerization domain (NOD)-like receptors, led to the understanding that these cells are not only under the control of accessory cells, but can be directly involved in the antibacterial response thanks to their capacity to recognize pathogen-associated molecular patterns. Interferon (IFN)-γ is the predominant cytokine produced by activated NK cells. IFN-γ is a key contributor to antibacterial immune defense. However, in synergy with other inflammatory cytokines, IFN-γ can also lead to deleterious effects similar to those observed during sepsis. Accordingly, as the main source of IFN-γ in the early phase of infection, NK cells display both beneficial and deleterious effects, depending on the circumstances.

Natural Killer Cells: A Coherent Model for Their Functional Role in <b><i>Mycobacterium tuberculosis</i></b> Infection

“…The second relies on the inhibitory recognition of self-proteins that are expressed by normal cells but down-regulated by infected or transformed cells (‘missing self-recognition') [15]. One of the most interesting advances in the biology of NK cells over the past few years came from demonstrating that they can express ‘pathogen recognition receptors', suggesting that a third strategy by which NK cells may undergo the activation and expression of effector functions is the direct recognition of ‘pathogen-associated molecular patterns' (PAMPs) [16,17]. In this context, the pioneer work of Chalifour et al [18] is particularly significant.…”

“…Finally, the ability of NK cells to positively regulate CD8 + and γδ + T cell functions while suppressing Treg expansion may suggest that they are somehow involved in the maintenance of the delicate balance between the effector and regulatory arm of the antigen-specific immune response. Although experimental evidence supports the view that NK cells may contribute to the immune response to MTB at several levels, the possibility that they might also be detrimental as a consequence of excessive activation should not be disregarded [16]. …”

TAP deficiency is also a cause of bronchiectasis

“…Although this is correct, Natural Killer (NK) cells might also be mentioned, as their role in the immune response to bacteria (besides their antiviral and antitumor functions) has been repeatedly documented 2 3. NK cells participate in innate immune defence mechanisms through (i) the release of cytotoxic granules containing, among others, the antibacterial factors granulysin, α-defensins and cathelicidin,2 and (ii) cytokine production that stimulates both innate and adaptive immunity. In addition, they have immunoregulatory properties able to limit excessive immune responses 3.…”

Influence of a probiotic Lactobacillus casei strain on the colonisation with potential pathogenic streptococci and Staphylococcus aureus in the nasopharyngeal space of healthy men with a low baseline NK cell activity

“…For example, in acute infections, probiotic intervention was able to induce total and pathogen-specific IgA [16], and an orally applied probiotic was able to induce a mobilisation of intestinal cells and to provoke changes in the cytokine profile, finally modulating the respiratory mucosal immunity in experimental viral infection [56]. In another study of Villena et al [57], a protective effect of probiotic intervention against the respiratory pathogen Strep. pneumoniae modulated respiratory mucosal immunity after local, intranasal application of the probiotic.…”