We sought to identify a clinically-relevant and easily measured parameter that may give insights into the status of the functionality of the immune system in HIV-infected persons that is independent of the contemporary means used to assess HIV disease stage, namely absolute CD4 T-cell counts. The results of this study indicate that among HIV-infected individuals who receive HBV vaccine after HIV diagnosis, the overall rate of experiencing an initial AIDS or death event was approximately 4-fold higher in vaccine non-responders than responders. Additionally, in a multivariable model, non-response to the vaccine characterized as an anti-HBs of <10 IU/L independently predicted a nearly 2.5-fold increase in the risk of developing clinical AIDS or death. Notably, this association was independent of CD4 cell count, VL, HAART use, duration of HIV infection, HIV seroconversion status, and the previously used marker of immune function, cutaneous DTH to recall antigens. Consistent with these observations, the event rates were significantly different between HBV vaccine non-responders and responders in those with CD4 cell count ≥500 cells/mm3 or with normal DTH responses. Collectively, these data indicate that the antibody response to HBV vaccine is tracking a unique aspect of immune function that cannot be optimally captured by assessment of CD4, VL or DTH responses to recall antigens. Furthermore, because HBV response serves as an independent prognosticator of clinical outcome in HIV infected persons, it may have clinical utility in the risk stratification of HIV-infected persons, including those with CD4 cell count ≥500 cells/mm3. Finally, as HBV vaccine responses predicted risk of AIDS or death independent of VL these results illustrate further importance of VL-independent determinants of HIV disease outcomes.
The exact mechanisms responsible for non-response to hepatitis B vaccine in HIV-infected individuals are not entirely known. Conceivably, the genetic and immune factors which underlie the non-response to HBV vaccine in HIV-uninfected persons are also present in those with HIV 
. However, in HIV-infected adults, additional mechanisms may be operative as the rate of non-responsiveness to HBV vaccine is significantly greater. In viremic HIV-infected individuals the interaction between CD40 ligand on CD4 T-cells and CD40 on B-cells (a key interaction for HBV vaccine responsiveness 
) is impaired 
. This reduction in helper T-cell function may at least partially explain the poor HBV vaccine responses observed in those with detectable VL. However, this T- and B-cell interaction appears to normalize following HAART 
. Therefore, the persistently poor HBV vaccine responses seen in those on suppressive HAART 
suggest that other aspects of immune function required for a response to HBV vaccine remain abnormal in aviremic individuals. Increased CD4 T-cell activation was associated with reduced responses to HBV vaccine and other similar neoantigens in subjects receiving HAART 
. Additionally, non-response to HBV vaccine in HIV-infected individuals was found to be significantly associated with an increased proportion of regulatory T (Treg
) cells 
cells have also been associated with non-response to HBV immunization and other vaccines in HIV-uninfected adults 
. Lastly, HBV vaccine non-response may reflect increased underlying immune senescence, which has also been associated with lack of responsiveness to other vaccines in healthy and HIV-infected adults 
While the mechanisms discussed above may explain the poor response rates to HBV vaccine in HIV-infected individuals, it is unclear which mechanism(s) may be specifically associated with the risk of AIDS or death. Decreased CMI is the hallmark of HIV-associated immune dysfunction 
, and responses to HBV vaccine are dependent on T-cell help 
, but we found HBV vaccine responses were associated with the risk of AIDS/death independent of CD4 cell count, and a marker of CMI, DTH responses 
. Furthermore, the independent association between HBV vaccine responses and AIDS/death was observed in the subset of those with CD4 count ≥500 cells/mm3
. Thus, facets of immune function other than CMI (as assessed by DTH) may also be associated with worse HIV clinical outcomes, and such aspects of immune dysfunction appear to be evident in those with relatively high CD4 cell counts. Immune activation has clearly been associated with HIV progression 
, so perhaps non-response to HBV vaccine serves as a marker of immune activation. Alterations in B-cell function are also well described in HIV and some do persist following HAART 
, but whether such impairments are associated with the risk of AIDS or death is not known. Lastly, while the complete role of Treg
cells in HIV pathogenesis and vaccine responsiveness remains undefined 
, immune senescence has been associated with more advanced HIV infection 
and increased risk of death in non-HIV-infected individuals 
. Therefore, further investigation of the mechanisms responsible for non-response to HBV vaccine may yield additional insights on aspects of immune function that are associated with clinical outcomes and which remain persistently abnormal or are restored following HAART.
One potential criticism of the current study may be that it included those that received less than the recommended three doses of HBV vaccine. We chose to include those with 1–2 doses of vaccine with the rationale that those who were able to respond to one or two doses of vaccine may have similar, or even better, immune function than those who responded only after 3 or more doses. The profound difference in the rates of AIDS or death for responders and non-responders observed in those with either 1–2 or ≥3 doses () suggest inclusion of those with less than 3 doses did not confound the results. Additionally, the number of vaccine doses was not associated with the risk of AIDS or death in preliminary multivariate Cox regression models, so it was not included in the final models. Another criticism may be that we used too wide of a window (1–12 months) to characterize vaccine response, allowing some responders to wane, thus introducing misclassification bias. While this is possible, such bias would only favor the null hypothesis, thus the difference in outcomes we observed is likely valid.
There were additional limitations to the current study. The HIV cohort analyzed in this study is distinct in some aspects when compared with other large HIV cohorts which may limit generalizability of the findings. Such characteristics include enrollment early after HIV infection due to military HIV screening, open access to care in the military health system, and virtually no intravenous drug use 
. However, these characteristics should only minimize confounding from drug use or differences in access to care, both of which can impact HIV outcomes 
. The current study does not apply to all HIV-infected individuals. Many patients with HIV infection do not receive HBV vaccine, as approximately 50% may already be HBV infected at the time of HIV diagnosis 
. Additionally, an increasing proportion of newly diagnosed HIV patients have received HBV vaccine prior to HIV infection 
. Additionally, the results of the current study may have been confounded as the majority of non-responders were vaccinated in the pre-HAART era, whereas, a higher proportion of responders were vaccinated in the HAART era and receiving HAART at the time of last vaccination. However, the consistency of the results from multivariable models adjusting for era of HIV diagnosis, and HAART use, with the sub-group analysis of patients not receiving HAART suggests that such confounding, if present, was minimal and did not diminish the overall study conclusions. Finally, VL and DTH responses were not available for all participants, and few events occurred in those on HAART and with VL≤400 copies/mL.
In conclusion, the antibody response to HBV vaccine may provide a measure of immune status that cannot be fully captured by assessment of the CD4 count, VL or DTH skin test responses, and as such, may serve as a tool for risk stratification of HIV-infected persons, including those with relatively preserved CD4 cell count. Understanding the reasons for HBV non-response in those with HIV may provide new insights into immune dysfunction during HIV, as well as result in the development of new immune-based therapies.