In our recent study, we have used the DotScan antibody microarray technology to identify differential cell surface markers expressed on CD4+ and CD8+ T cells between 3 HIV disease groups and uninfected controls 1. Along with confirming the cell surface markers previously described in the context of HIV disease, we identified 5 novel markers that could segregate HIV disease stages. This study together with the study by Woolfson et al., who used a similar antibody microarray to show the conservation of unique cell surface antigen mosaics in cryopreserved PBMCs from HIV+ individuals 2, demonstrated the power of this technology as an adjunct to flow cytometry in HIV research. Even though T cell subsets could provide more specific information, as evident from our previous study 1, PBMCs have already been shown to be acceptable as starting material for antibody microarray analysis of HIV disease status as well as for classifying leukemia types 23.

During the natural course of HIV infection, the major determinant of the depletion of CD4+ T cells is immune activation 4. Several previously described surface markers are up-regulated on T cells during the activation process, and are known to have a profound effect on the course of HIV disease 4. Importantly, progression of HIV infection correlates with increases in circulating markers of immune activation such as soluble interleukin-2 receptors (sIL-2R) 5, soluble tumor activation markers such as necrosis factor receptor type II (sTNF-RII) 6 and monocyte activation markers such as neopterin 7. Recently, a few new cell surface markers involved in HIV pathogenesis and disease progression have been identified. These include CD137L (4-1BBL), which was shown to be a critical component in the rescue of functionally impaired HIV-specific CD8+ T cells 8; CTLA-4, the inhibitory immunoregulatory receptor, whose expression correlated positively with disease progression and negatively with the capacity of interleukin 2 production by CD4+ T cells in response to viral antigen 9; and PD-1 on HIV-specific T cells, the inhibitory receptor programmed death 1, whose expression was associated with T-cell exhaustion and disease progression 10.

The advent of HAART has led to a dramatic decline in AIDS-related morbidity and mortality by decreasing plasma viremia and increasing CD4+ T cell counts 1112, normalizing the progenitor cell function 13 and restoring CD4+T-cell functions 1415. In treatment-naive individuals who initiate HAART and can attain complete viral suppression, T cell activation declines as plasma viremia decreases 16. Treatment failure appears to be associated with increases in T cell activation and rapid decline in CD4+ T cell numbers. In contrast, T cell activation appears to decrease in patients attaining good control of viral replication while on HAART, and is maintained at low levels during the prolonged periods of complete viral suppression 17. In some patients achieving suppression of viremia, T cell activation may still be evident. This may be attributable to residual viral replication, and this may affect the extent of CD4+ T cell recovery during HAART. Although HAART's ability to reduce viral load to below the detection levels has been well documented, the mechanisms involved in the immune reconstitution resulting from this treatment are still not fully understood. A thorough characterization of changes induced by HAART on the broad immunenophenotype of the immune cells over time may facilitate the clarification of these mechanisms.

Although a considerable amount of work has already been done to elucidate surface marker modulation during HIV disease and therapy by flow cytometry, this study is the first to use a cell-based antibody microarray (135 antigens) to retrospectively and longitudinally monitor the effect of antiretroviral therapy on cell surface antigen expression using frozen PBMC over time. Two HIV+ groups were studied: sustained responders (SR) who achieved sustainable response by maintaining below detectable plasma viremia on HAART and transient responders (TR) who responded intermittently to HAART. Our hypothesis is that modulation of cell surface markers occurs during the course of HIV disease and following the initiation of HAART and these cell surface markers may indicate the outcome of antiretroviral therapy. Along with confirming the cell surface markers previously described, we aimed at identifying novel potential cell surface markers associated with HIV disease progression and HAART efficacy.

Using DotScan technology, we have recently carried out a cross-sectional study to demonstrate that HIV disease stages can be segregated by cell surface antigens on CD4+ and CD8+ T cells 1. The present study, to our knowledge, is the first retrospective longitudinal study using antibody microarray to monitor the effect of HAART based on CD marker expression using frozen PBMC. Simultaneous analysis of 135 cell surface antigens on PBMC from 12 HIV+ patients on HAART was performed over a two year period, and the patients were stratified into sustained responders and transient responders. Our study not only demonstrated potential associations between modulations of cell surface antigens and activation or restoration of the immune system, but also identified markers segregating sustained and transient responders to antiretroviral therapy, as well as markers significantly correlating with CD4+ or CD8+ T cell counts.

The majority of antigens which showed a trend over time for combined patient groups were associated with cell activation, implicating a general immune activation status of PBMC from patients on HAART. Notably, this activation was initially controlled during the first year, but was ultimately elevated after two years of HAART therapy. For instance, after 2 years of HAART, the CD3epsilon, and two co-stimulatory molecules CD27 and CD28 were upregulated relative to the baseline after an initial period of stability for the first 12–18 months. In HIV-infected individuals, the primary signal through TCR/CD3 is decreased, though response to costimulation through CD27 and CD28 is relatively preserved 22. These stimulatory signal receptors were all increased during HAART, possibly as a consequence of the partial restoration of the impaired T cell responses during HAART.

CD95, CD2 and CD5 also showed a pattern similar to what was observed for CD3epsilon. The increase in CD95 expression over time was consistent with a previous study, which showed lack of control of T cell apoptosis under HAART 23. CD2 mediates both cell-to-cell adhesion and T cell activation; also the CD2/LFA3 pathway may cooperate with signals from the TCR pathway to amplify HIV expression in vivo 24. The biological relevance of increased CD5 expression is unclear though it has been suggested that up-regulation of CD5 on T cells can be a physiological event depending on protein kinase C activation 25. Alternatively, the increase in CD5 binding may reflect the restoration of CD5+ T cell numbers in HAART treated individuals. Although HAART prolongs the period of controlled T cell activation, the observed elevation of activation markers over time indicates the eventual failure of HAART to control the chronic immune activation caused by HIV infection. It is thus plausible to hypothesize that during the initial stage of HAART therapy (up to 1.5 years in our study), substantial decreases in HIV antigen lead to the transient lowering of immune activation. However HAART eventually fails to control low level replication in HIV reservoirs, which is possibly responsible for continued cellular activation at the later stages of therapy, even when the viral load remains below detection.

The demonstrated increases in CD36 expression over time may be associated with the lipid metabolism derangements caused by HAART. In support of our findings, which showed increase in CD36 expression over time during HAART, Dressman et al., 26 showed that CD36 plays a crucial role in cellular uptake and accumulation of lipids, and protease inhibitors induce a specific increase in macrophage CD36 levels, which may promote accumulation of sterol in macrophages, foam cell formation and atherosclerosis 26. Increased CD36 expression has also been found on circulating monocytes during HIV infection, which could represent a proatherogenic condition in HIV-infected patients 27. Although the mechanisms regulating CD36 expression during HIV infection and HAART remains to be elucidated, it is imperative to carefully evaluate the role of CD36 expression especially during HAART as this treatment is known to be associated with increased cardiovascular risk, hyeprlipidemia and lipodystrophy in HIV patients. Surprisingly we didn't observe any statistically significant trend over time for CD4 and CD8 expression. But compared to the pre-therapy time points, the median of CD4+ T cell counts increased slightly while CD8+ T cell counts decreased slightly (Table 1). The lack of significant trend may be due to too many overlapping values in the cell counts between the time points and this trend may be detected by enlarging the sample size and increasing the time points.

Our study also found that five cell adhesion molecules (CD11a, CD11c, CD44, CD49d, CD49e) might serve as surrogate markers for disease progression, since the changes in expression levels of these molecules were highly correlated with the changes of either CD4+ or CD8+ T cell counts (p < 0.001). To our knowledge, this is the first report of a clear correlation between adhesion molecules and CD4+ and CD8+ T cell counts, though altered CD11a, CD44 and CD49e expression on cell subsets during HIV infection or disease progression has previously been reported 2829. Although the biological significance of the adhesion molecules remains largely unknown, it has been suggested that the plasma levels of several soluble adhesion molecules (CD11b and CD54) may have a potential application in assessing prognosis and efficacy of the HAART 30. Therefore, the relationship between patient response to HAART, cell surface expression of adhesion molecules and levels of circulating adhesion molecules requires further investigation.

Three cell surface antigens were associated with cell activation (CD9, CD27 and CD28) positively correlated with the CD4+ T cell counts. Previous studies may provide some clues to the mechanism underlying these correlations: the overexpression of CD9 rendered cells less susceptible to HIV envelope-mediated syncytia formation 31, the expression rate of CD28 on CD4+ T cells was positively correlated with CD4+ T cell counts 32, while plasma soluble CD27 was inversely correlated to CD4+ T cell counts 33. A negative correlation between CD52 expression level and CD8+ T cell counts was observed. It has been shown that CD52 expression may be associated with the resting state of T cells 34.

The reliability of the antibody microarray technology was further confirmed by the observation that the CD4+ T cell binding density measured by antibody microarray was significantly correlated with both CD4+ and CD8+ T cell counts measured by flow cytometry, with adjusted p < 0.001 and 0.042, respectively.

Our study is unique in identifying 10 cell surface antigens, whose expression levels distinguished between sustained and transient responder groups, which have implications for the evaluation of HAART efficacy. The mean values for CD11a, CD29, CD38, CD45RO and CD52 binding were significantly higher in the SR group at all time points than those in the TR group, while the mean values for CD56, CD57, CD62E, CD64 and CD33 were significantly lower. Although the biological relevance of the changes observed in these antigens needs further investigation, many of these molecules have already been implicated in HIV infection. CD38 and CD45RO are well documented cell activation markers. CD11a expression on lymphocytes has been shown to be related to clinical stage of disease 35, while CD29 (β-1 integrin chain) is involved in the regulation of an inflammatory effector gene 36. CD56 is a NK-associated marker and its expression on CD8+ T cells identifies the mature cytolytic effector cells 3738. CD57 expression on CD8+, CD4+ T cell and NK cells is a general marker of cell proliferative inability and senescence 39. CD64 (FcgammaRI) was involved in FcgammaR-mediated phagocytosis, which is impaired by HIV-1 infection in monocyte-derived macrophages 40. Although the biological roles of CD62E (E-selectin) and CD33 are unknown in the context of HIV infection, the plasma levels of CD62E has been proposed for monitoring disease activity in patients with chronic inflammatory syndromes 4142 and CD33 expression was significantly increased on alveolar macrophages of HIV+ patients compared with healthy controls 43.

Interestingly, this longitudinal study and our recent cross-sectional study 1 have detected 3 cell surface antigens in common (CD3epsilon, CD9 and CD57). This coincidence may imply that these markers have some crucial roles in HIV disease and HAART. Another notable feature is that both studies have pointed to the importance of cell adhesion molecules in disease progression. Although adhesion molecules have been reported in HIV disease, the biological relevance of most of these molecules is not well understood. Our study provides a strong foundation for understanding biological relevance of most of these molecules through further investigation.

Our findings not only have implications for the evaluation and future direction of HAART, but also show how in an array-like manner the density of cell surface antigens could be efficiently exploited in studying cell-surface modulation during HAART and HIV-infection. Such investigations would be labor-intensive, time-consuming and expensive if done by flow cytometry. Secondly, the detections of cell surface antigens in our study lay a solid foundation for future functional assessment of these markers. The differential antigens identified in this study should be further investigated by other methods such as flow cytometry for confirmation since DotScan technology does not distinguish between modulation of antigen expression and changes in the proportion of cell population expressing the antigen. A biological analysis of these markers may also help to clarify their role and may lead to the discovery of new biomarkers for HIV prognosis/diagnosis. Further investigation on detailed subset composition of CD4+ and CD8+ T cells should be able to provide more specific information related to immunoreconstitution under therapy since this study cannot differentiate the changes of CD4+ or CD8+ T cell subsets, which may have direct impact on the cell immunophenotype.

Abbreviations used in this paper: HAART: Highly active antiretroviral therapy; SR: Sustained responder; TR: Transient responder.

The authors declare that they have no competing interests.

JQW fully performed the work, analyzed data and wrote the paper. WBD contributed to vital patient samples and immunological interpretation of findings and BW contributed to the writing. LB and JC analyzed data, did statistical evaluation, contributed to the technology and the writing. NKS designed the research project, supervised this work and contributed to the writing. All authors read and approved the final manuscript.