NK cells have a pivotal role in the innate immune response against viruses and cancer. However, studies on the role of NK in anti-TB immunity are limited. In order to investigate if Mtb infection modifies NK cell phenotypic signatures, we estimated circulating NK subsets, as well as their expression of KLRG1 and NKG2D in different TB-infected status. Significant different of NK phenotypes were observed among the ATB, LTBI and HC populations. Compared with HC group, CD56BrightCD16Dim and CD27 + CD56BrightCD16Dim NK were increased in TB-infected groups. Furthermore, LTBI individuals showed significantly increased KLRG1 + NK cells than ATB patients, while Mtb specific IFN-γ + CD56BrightCD16Dim NK cells expressed higher KLRG1 in ATB than LTBI. Our study adds novel information on KLRG1 expressed NK cells, which might be a specific phenotype to modulate the progression of TB from latent to active.
Encouraged by the hypothesis that NK cell function and subsets in the blood can inform Mtb infection status , circulating NK subsets from ATB, LTBI and HC cohorts were analyzed by flow cytometry. In line with other evidence [22, 25], the frequency of CD3 − CD56 + NK cells were similar in the three participant groups. Generally speaking, NK functions can be preferentially assigned to two major NK subsets, with the CD56DimCD16 + subset being intrinsically highly cytotoxic and the CD56BrightCD16Dim subset being more efficient to produce cytokines, mainly IFN-γ . By releasing IFN-γ, the CD56BrightCD16Dim NK cells are assumed to provide protective mechanisms in LTBI subjects . However, we observed that the percentage of CD56BrightCD16Dim NK cells were significantly increased in both ATB and LTBI groups. Although NK functions tend to be exhausted during the long-term progression of TB [28, 29], the abundance of CD56BrightCD16Dim subsets in TB-infected groups probably reflects their important role in the early Mtb responsive accessory cells.
NKT, along with NK, is known as the bridge between innate and adaptive immune response. The role of NKT cells in the context of Mtb infection is still controversial. Consistent with the results of Pandey et al. , we found that NKT cells were elevated in ATB patients compared with HC individuals, which further indicated the importance of innate immunity to against TB.
Given that NK cell surface marker redistribution has been described in the setting of some infections [31, 32], typical activating and inhibitory receptors expressed by NK subsets, and their ability to secrete IFN-γ were studied in each of the three participant groups. KLRG1 is widely used as a lymphocyte differentiation marker in both humans and mice but the physiological role of KLRG1 in TB is still unclear. Our results revealed that KLRG1 expressing NK cells, as well as their subsets CD56DimCD16 + and CD27 + NK cells were reduced in ATB patients compared with LTBI individuals, which was similar to another report on the role of KLRG1 in human CD4 + T cells . There is undoubtedly better immune control in LTBI, our results indicated that KLRG1 + NK was an important component affecting the balance between host immunity and Mtb to keep TB latent. Whereas Hu et al. reported that KLTG1 expression in CD4 + T cells was significantly increased in ATB patients compared with HC subjects , the difference in research objects could partly explain the above discrepancy. Nevertheless, when compared with HC group, the percentage of KLRG1 + CD56BrightCD16Dim NK subsets was also elevated in ATB patients. In the above study, KLRG1 + CD4 + T cells tended to produce more IF-γ than KLRG1 − counterparts in ATB patients. However, IFN-γ was found highly secreted on KLRG1 − NK cells compared with KLRG1 + counterparts in the present study. Through literature search, little information is currently available about the expression of KLRG1 in NK cells, especially about its role in NK subset function during TB infection. Nevertheless, similar results have been confirmed in other studies on HCMV infection . In addition, within Mtb specific IFN-γ + CD56BrightCD16Dim NK cells, the expression level of KLRG1 in ATB patients was higher than that in LTBI population, which reflected the trend of NK cell function depletion during TB progression. In brief, the above results indicated that KLRG1 as an inhibitory receptor of NK cells plays an important role in anti-TB immunity, and circulating KLRG1 + NK cells may reflect the difference of host immune status between ATB and LTBI. Future studies evaluating the role of KLRG1 + NK cells during the course of TB are warranted.
Interestingly, the activating receptor NKG2D did not significantly affect NK diversity during TB infection, with no significant changes were observed in the participant groups. Normally, NKG2D are involved in the NK cell mediated lysis of human Mtb infected monocytes by enhancing the production of perforin and granulysin ; we found that NK cells expressing NKG2D also had a stronger IFN-γ secretion capacity compared with NKG2D − subjects.
The CD27 marker is a member of the TNF-receptor superfamily, and its low expression in IFN-γ + CD4 + T cells has been proposed as an active TB marker [35, 36]. Although the CD27 expression varies on murine NK cells , little is known about the expression of this receptor in human NK cells. We here showed that CD27 was expressed differently in NK subsets based on CD56 and CD16 typing. Generally, the expression of CD27 is associated with immunoregulatory and cytokine producing . Consistent with the above statement, CD27 was found mainly expressed in CD56BrightCD16Dim NK cells and up regulated in TB-infected groups, which explained the response of ATB and LTBI individuals to secrete IF-γ against Mtb. Furthermore, the phenomenon that CD27 + NK subsets expressed higher NKG2D than CD27 − subsets further demonstrated our conclusion above that NKG2D + NK cells tended to secrete more IFN-γ than negative cells.
This study also had some limitations. First of all, it was a single center study, and the statistical power of analysis may be insufficient due to the limited sample size. Secondly, other activating and inhibitory receptors, such as NKp46 and the KIR family; as well as NK function markers, such as CD107a and granzyme B, were not systematically evaluated in the presents study. Finally, the fluctuations of NK subsets during TB treatment were not monitored as well. Further researches are needed to fill these gaps.