Here we confirm previous reports of increased NK activation in exposed uninfected subjects, and show for the first time that increased PDC maturation is also a marker of the heightened innate immune activation state observed in EU-IDU subjects. Despite a state of persistent activation, we show that both PDCs and NK cells from EU-IDU maintained strong effector cell function and did not exhibit signs of exhaustion. Overall, our results highlight the role of PDCs in augmenting NK function and show how both members of the innate immune response are co-modulated during high-risk activity in EU-IDU subjects.
Based upon previous findings indicating an enrichment of protective NK KIR3DL1high
receptor alleles in high-risk HIV-1 exposed, uninfected subjects [16
], we correlated NK phenotype and function with genotypic analysis of KIR
and their HLA-Bw4
ligand alleles. Our findings indicate that inheritance of protective KIR
alleles or their corresponding HLA
ligands was not enriched in our cohort of EU-IDU subjects compared to control uninfected donors from the same geographic area. Rather, our results indicate that increased maturation of PDCs is associated with heightened NK activation in EU-IDU subjects. However, a greater number of subjects will be required to draw firm conclusions regarding KIR
allele frequency and resistance to infection at the population level in high-risk subjects.
Our observations are of interest in light of the controversial role of opioids on NK cell function where contrasting data suggests that opioids can inhibit NK activity or can lead to a “tolerant” state that is dependent on frequency of drug use and timing of NK analysis relative to drug exposure [40
]. Based upon the frequency and duration of injection drug use among the EU-IDU subjects in our cohort (see ), we speculate that the innate immune response of EU-IDU subjects reaches an equilibrium state after years of prolonged injection-drug use. Indeed, work from several groups supports a loss of innate function during acute exposure to opioids or upon opioid withdrawal, rather than chronic opioid usage [43
]. As has been described previously for other cohorts of EU-IDU subjects [19
], we observed that NK cells from EU-IDU maintained strong effector cell function when stimulated in vitro
(). We now extend these observations to include PDC cells from EU-IDU subjects, which remained functional in vitro
() despite evidence of heightened PDC maturation ex vivo
It is also important to note that while high-risk drug-use is a hallmark of the EU-IDU subjects, approximately half of the subjects from our cohort also reported a history of recent high-risk sexual activity. This included a high frequency of sexual events without the use of a condom as well as a high number of sexual partners over the previous three-month period (). Interestingly, we did not observe any correlation between the frequency of high-risk sexual practices or the number of sexual partners and the extent of innate immune activation in EU-IDU from our cohort. Future work will be needed to determine to what extent sexual exposure (if any) can enhance the observed innate immune activation phenotype among IV drug users as our data does not allow us to specifically address this point.
In addition to the influence of opioid usage on innate immune function, the covariable of HCV exposure in injection drug users proved informative in interpreting the functional data from our cohort. As described, we observed an impairment in the ability of the HCV sero-positive subjects to secrete IFN-α following CpG-ODN 2216 stimulation (). However, PDC-dependent NK activation was similar between HCV sero-positive and HCV sero-negative EU-IDU subjects suggesting that PDC/NK crosstalk is sustained in EU-IDU despite evidence of HCV infection (). Interestingly, the increase in PDC maturation we observed among EU-IDU subjects was independent of HCV status, as both HCV sero-positive and HCV sero-negative EU-IDU subjects exhibited increased CD83 expression compared to controls (). This finding suggests that HCV sero-positivity is not a requirement for the heightened PDC maturation observed in many EU-IDU subjects. Nevertheless, the high incidence of HCV sero-positivity in our cohort of EU-IDU (10/14, 71%) suggests that persistent exposure to HCV virus is likely encountered during prolonged IV-drug use and may contribute to PDC maturation.
Overall, our results highlight NK cells and PDCs as candidate cell types whose retained function and heightened activation status may contribute to resistance upon HIV-1 exposure.