This study compared proportions of NK cell subsets as well as expression of activating receptor NKG2D and cytotoxicity receptor NKp46 among adults with ‘suboptimal’ and ‘super-optimal’ immune recovery despite four years of suppressive HAART. We found that CD56dim was the largest NK cell subset among HAART-treated adults, irrespective of immune recovery status. Our data is consistent with previous reports that CD56dim is the largest population of NK cells in peripheral blood and the main cytotoxic NK cells participating in Antibody-dependent cell-mediated cytotoxicity (ADCC); followed by CD56neg and CD56bri NK cells. Whereas the functionally defective CD56-CD16+ (CD56neg) population of NK cells expands in viremic versus aviremic patients and is associated with poor cytotoxic function, this subset of NK cells was lower among suboptimal responders relative to super-optimal responders. Given that our study participants had received suppressive HAART for four years, our data suggests that HIV-associated expansion of the dysfunctional CD56neg population was no longer significant. These results mirror previous reports that initiating HAART during acute HIV infection prevented further decline in NK cell subsets and improved NK cell function. It is therefore likely that initiating HAART earlier in HIV disease when the immune systems are still robust, as recommended in the 2013 WHO guidelines, may result into faster recovery of HIV-associated NK cell dysfunction; among other benefits.
The CD56++CD16- (CD56bri) subset, functionally cytokine producers, was higher among ‘suboptimal’ responders relative to ‘super-optimal’ responders. The high numbers of cytokine producing NK cells among ‘suboptimal’ responders may be reflective of the persistently high levels of immune activation that were previously documented among suboptimal responders in our cohort. Immune activation has been associated with high production of inflammatory cytokines and increased turn-over of T-cells, B lymphocytes, NK cells and accessory cells[27, 28]. Immune activation in the first few years of HAART-mediated viral suppression predicted long-term CD4+ T-cell recovery after 15 years of antiretroviral therapy. It is likely that pre-HAART immune activation, not only predicts mortality during HAART, but also predicts suboptimal immune recovery including suboptimal reversal of the HIV-associated NK cell dysfunction. We therefore postulate that controlling immune activation among HIV-infected individuals may also stabilize the NK cytokine producing cells, modulate their immune function and subsequently optimize short and long-term immune recovery during antiretroviral therapy.
Expression of NKG2D and NKp46 receptors by NK cells was comparable among suboptimal and super-optimal responders. NK activating receptors correlate with the NK effector function[30, 31], so it is likely that NK effector function is comparable among suboptimal and super-optimal responders. We could attribute this result to suppressive HAART that has provided partial immune recovery during the first four years. It has been previously shown that HAART modulates NKG2D receptor expression among HIV-infected viremic individuals[10, 13, 21]. Increased NKp46 expression on NK cells was previously shown to correlate with HIV-1 disease severity among HIV-infected children, although its role in immune recovery is not yet well understood. Although we did not perform NK functional assays, previous data shows that abnormal expression of NK activating and inhibitory receptors was associated with impaired cytolytic function. In addition, reduced surface expression of the NK cytotoxicity receptor, NKp46 was associated with poor cytolytic function during viremic HIV disease. Given that NKG2D and NKp46 expression was similar in the ‘suboptimal’ and ‘super-optimal’ immune responders, it is likely that these receptors are not involved in the mechanisms that lead to poor CD4+ T-cell reconstitution in HIV-infected adults receiving HAART unless functional assays reveal significant differences. In addition, NK cell activation is controlled by a dynamic balance between complementary and antagonistic pathways[31, 32]. We did not evaluate NK cell surface inhibitory receptors that antagonize activating pathways through protein tyrosine phosphatases (PTPs), therefore our data is not conclusive on the NK cell activation status in the study population.
Implications of the study
Our results imply that after four years of suppressive antiretroviral therapy, the HIV-associated NK cell dysfunction was only partially restored, with a predominant CD56dim (CD56 + CD16-/+) population and a high CD56bri (CD56++CD16-) NK cell population among suboptimal responders. The high CD56bri, functionally cytokine producers, among suboptimal responders may be reflective of the persistently high levels of immune activation previously described in the same cohort. We postulate that earlier initiation of HAART and control of immune activation could contribute to faster and more comprehensive recovery of the immune system. It is also important to note the trend shown that specific T-cell subsets recover faster, while other subsets require longer periods of suppressive HAART. Given the significant cytokine producing function of CD56bri NK cells, it might be worthwhile to further investigate NK cell dysfunction among individuals that initiate HAART at CD4 < 500 cells before severe damage of the immune system, as well as potential interventions to enhance comprehensive immune recovery. In addition, increased NK cell degranulation capacity was significantly associated with Immune Reconstitution Inflammatory Syndrome (IRIS) among HIV/TB co-infected individuals in Cambodia, with activating receptor expression higher among IRIS patients relative to non-IRIS patients. Similarly NK cell activation was shown to distinguish Mycobacterium tuberculosis-mediated IRIS from chronic HIV and HIV/TB co-infection. Thus, further examination of the mechanisms of NK cell dysfunction, co-infections and suboptimal recovery is critical for suboptimal immune responders that remain at risk of life-threatening opportunistic infections[1, 24].
We did not perform NK cell function assays due to logistical limitations. In addition, this study was limited to the extremes of immune recovery (suboptimal and super-optimal immune responders), and did not include average responders. Our results, however, highlight the need for NK function assays to conclusively ascertain defects in NK cell effector function that might be relevant to immune responses to viral and bacterial infections among HAART-treated HIV-infected adults.