The inability to reliably predict genital shedding of HIV and other sexually transmitted viruses represents a public health problem. Understanding the viral and immunologic dynamics in the genital tract and characterizing the factors that increase the risk of sexual transmission could provide important information for the development of effective prevention strategies. In this study, we investigated the interactions between CMV, HSV, and HIV replication in the male genital tract and the associated lymphocytic changes, locally in semen and systemically in blood.
Similar to previous studies (20
), HIV-infected participants in our cohort had higher levels of CMV in semen than HIV-uninfected controls. Unlike historical reports, however, our HIV-infected participants had a lower proportion of semen samples with detectable CMV DNA: 38% versus 56 to 66% (22
). Most likely, this is a consequence of the higher average CD4 cell count found in our study participants (43
), who are also likely more representative of the current HIV-infected population in the United States (34
). As might be expected, CMV DNA was undetectable in all of our HIV-infected participants in blood plasma, despite frequent detection of very high levels of CMV replication in semen (median of 4.8 log10
copies CMV DNA/ml seminal plasma among positive samples). Such compartmentalized replication of CMV in the genital tract, first described in 1972 (45
) and subsequently confirmed by others (11
), probably has consequences for CMV transmission (11
), impacts localized immune responses, and interacts with other sexually transmitted viruses, like HIV-1 (66
Our investigation of localized immune reactions showed that HIV-infected participants had higher numbers of seminal CD3+
T lymphocytes and higher levels of activated CD4+
lymphocyte subsets compared to HIV-uninfected groups. HIV-negative MSM had higher proportion of seminal CD8+
T cells, higher seminal CD8+
T-cell activation, and significantly higher activation levels of both CD8+
T cells in blood than HIV-negative MSW (). As reported by others (41
), the higher immune activation seen in blood among MSM controls may be secondary to increased antigen exposure. This is consistent with what is reflected also in the higher seroprevalence of viral infections and STI in our MSM control group, but further evaluation is needed.
As previously described (19
), our HIV-infected participants had fewer CD4+
T cells in semen than the HIV-uninfected group, which is consistent with findings at other mucosal sites (10
). In the male genital tract, low numbers of CD4+
T cells, which are the primary target cells for HIV replication, may explain the inefficient rate of transmission seen for HIV compared to other STI (62
In contrast to our findings, a prior study (57
) reported significantly lower concentrations of total leukocytes, including CD4+
T-cell subsets and activated T lymphocytes in semen of HIV-infected men compared to HIV-uninfected individuals. Unlike our study, the prior study population mostly consisted of chronically infected patients with advanced HIV disease. In fact, when a subanalysis of the same study was performed to include only subjects with high peripheral CD4+
cell counts (>500/mm3
), a reduction in CD4+
T cells but not CD8+
T cells was the only difference between the HIV-infected and uninfected groups. This prior study also did not assess the cellular subset changes in relation to viral levels in the genital tract, which may have revealed additional mechanisms.
Although a recent study from our group suggested that virus transmitted during sexual exposure between MSM originated from seminal plasma (12
), data from other studies support the hypothesis that HIV-infected cells, deposited in the genital tract or rectal lumen during sexual intercourse, shelter and transport virus to susceptible cells within or below the mucosal epithelium during infection of a new host (reviewed in reference 5
). It is, therefore, possible that an accumulation of infected CD4+
T cells in semen together with enhanced cellular HIV transcription and replication in activated T lymphocytes (6
) could increase the risk of sexual transmission of HIV. Moreover, persistent immune activation of lymphocytes in semen and in blood may impact HIV disease progression and overall clinical outcome. To evaluate these hypotheses, we examined the correlations between the levels of CMV, HSV, and HIV and the absolute numbers and activation statuses of CD4+
T lymphocytes in semen and blood.
In multivariate analysis, seminal HIV levels were not associated with any of the measured parameters in semen (i.e., CD4+ and CD8+ T cells and CMV or HSV levels), and the only independent predictor for higher HIV RNA levels in semen was an increased proportion of activated CD8+ T cells in blood (P = 0.02). On the other hand, higher seminal levels of CMV correlated with increased number and activation status of T lymphocytes in semen and were an independent predictor of increased CD4+ T-cell activation in blood (P < 0.01).
The last observation is supported by previous findings that CMV replication in the male genital tract contributes to systemic immune activation (32
Taken together, these results suggest that CMV, much more than HIV, influences immune cell dynamics within the male genital tract and support a model in which localized CMV replication recruits T lymphocytes to the male genital tract and induces activation in these lymphocytes ().
Fig 4 Theoretical model showing interactions between CMV, T lymphocytes, and HIV in blood and semen. CMV replication in semen (part 1) drives T-cell recruitment and cellular activation, directly in semen (part 2) and indirectly through T-cell activation in (more ...)
Since chronic persistent immune activation negatively impacts HIV disease progression (18
) and immune activation in response to CMV may be responsible for accelerated immunosenescence (1
), CMV replication localized to the genital tract is likely a major factor in HIV disease progression. Alternatively, seminal CMV replication may be a proxy for increased CMV shedding in other tissues that were not sampled in this study.
Whether or not suppression of this localized CMV replication influences HIV-related disease progression or transmission is an open question.
Although these data are intriguing, there are a number of limitations to the current study. First, because this was a retrospective, observational study, we cannot establish a true causal relationship between the detected correlations. For example, it is possible that an untracked variable caused the observed increases in T-cell numbers and activation, which then triggered a reactivation of HIV, CMV, or both. However, consistent with our study results, a recent randomized study (36
) found a significant decrease of immune activation in the blood of HIV-infected individuals following treatment with valganciclovir, suggesting that CMV replication is likely a significant contributor to T-cell activation in the blood. A second limitation is that screening for STI was performed at baseline and only repeated after 3 months for some subjects. Unrecognized bacterial STI could, therefore, confound our results and might explain why some subjects had elevated seminal lymphocyte counts despite undetectable levels of any of the three measured viruses. We believe this scenario is unlikely, given that none of the subjects reported symptoms consistent with a bacterial STI. A more likely possibility is that another highly prevalent genital tract viral coinfection that was not evaluated, such as Epstein-Barr virus (EBV) or human herpesviruses (HHVs), caused the observed immunologic modulation. Future studies will need to be more comprehensive in evaluating the semen for all potential viral coinfections. Third, this study is limited by a relatively small sample size, especially for participants with HSV infection. However, the driver of genital tract and systemic immune activation at the population level is likely to be the more prevalent virus, i.e., CMV in populations with relatively low HSV prevalence.
To our knowledge, this is the first report describing in detail the interactions between CMV, HSV, HIV, and T-lymphocyte subsets and activation status in semen. Although our study was relatively small, both univariate and mutivariate analyses support the hypothesis that CMV replication in the genital tract drives lymphocyte recruitment and activation in both semen and blood. However, the virologic and immunologic relationships that exist in different anatomic compartments are likely too complex to fit into a single mechanistic model in which “CMV replication in the male genital tract equals systemic lymphocyte activation.” Despite its limitations, the present study provides some important insights with regard to (i) the interaction between CMV and HIV in the seminal compartment, which is likely to be important in the biology of sexual transmission of both viruses and (ii) the interactions between CMV replication in the genital tract and localized and systemic inflammation, which probably impact both HIV transmission and disease progression.