The major finding of the present study is that HIV-negative individuals with pulmonary TB, both with and without effusions, had greater seropositivity for anti-HHV8 antibodies than age- and sex-matched healthy controls. These results are in line with the results of our previous study of subjects with TB pneumonia [11]. The cutoff point for HHV-8 seropositivity in the present study was set at 1:40 according to the manufacturer’s instructions. In our recent study of HHV-8 seroprevalence in patients with end-stage renal disease, one of the two healthy controls with IFA antibody titers of 1:160 displayed positivity in a specific enzyme-linked immunosorbent assay (ELISA) [16]. If the cutoff point for seropositivity was set at 1:160 in our previous study, then the seropositive rate among TB patients was marginally higher (5/101 vs. 0/101; p = 0.059; Fisher’s exact test) [11], but in the present study the seropositive rate would remain significantly higher in patients than in controls (8/129 vs. 1/129; p = 0.036; Fisher’s exact test). This discrepancy might be due to fewer subjects in the previous study.
The present study demonstrated that the seroprevalence of HHV-8 was similar among TB patients with and without effusions (27.5 vs. 32.6 %). As noted in our previous study of subjects with pulmonary TB [11], HHV-8 seroprevalence in HIV-negative TB patients with pleural effusions was lower than in HIV-negative patients with KS (52–100 %) and comparable to that in HIV-positive patients without KS (13–50 %) [8–10, 17]. This suggests that HHV-8 infection has similar roles in TB patients with and without effusions. However, the study groups enrolled in the present and previous studies were much older than individuals who have the greatest risk for HIV infection.
Previous studies reported that the prevalence of HHV-8 infection increases with age [18–20], and that KS is more common in elderly males [21]. Our previous comparison of TB patients and healthy controls [11] indicated similar prevalence of HHV-8 seropositivity for males and females and similar mean ages for seropositive and seronegative subjects. In agreement, the present study demonstrated similar seropositivity for males and females, similar mean ages for seronegative and seropositive subjects, and similar seropositivity for TB patients with and without effusions.
Our previous study reported similar positivity for HHV-8 DNA in TB patients and healthy controls (4/101 vs. 0/101, p = 0.12) [11]. However, the present study indicated that HHV-8 DNA positivity was significantly greater in the TB group than in the control group (6/129 vs. 0/129, p = 0.03). These discrepant findings might be attributable to smaller number of subjects in our previous study.
Our previous study of patients with cirrhosis and ascites indicated the positive rate and titers of anti-HHV-8 antibodies were much greater in plasma than ascites (p < 0.0001) [22]. Among cirrhosis patients, the positive rate of anti-HHV-8 antibodies in plasma samples was more than 6-fold greater than in effusion samples. By contrast, the present study indicated similar positive rates and titers of anti-HHV-8 antibodies in plasma and effusion samples among TB patients with effusions (13/38 vs. 10/38, p = 0.45). The most likely reason for this difference is that TB effusions are exudates, but cirrhosis effusions are transudates [23].
HHV-8 is associated with PEL, a rare form of malignant clonal B-cell lymphoproliferation [12]. Among patients with PEL, the level of HHV-8 DNA in effusion samples is significantly higher than in blood samples [13]. In contrast, the present study found that plasma and effusion samples from the 38 TB patients with available effusions were all negative for HHV-8 DNA. This suggests that the role of HHV-8 is different for patients with TB and those with PEL.
The present study found that TB patients with or without effusions had lower lymphocyte counts and higher monocyte counts than healthy controls (p < 0.0001 for both), in agreement with our previous study of patients with TB pneumonia [11]. Stimulated pleural macrophages can induce production of interleukin-8 (IL-8) protein, and IL-8 in turn can induce lymphocyte chemotaxis into the pleural space [24]. Hence, lymphocyte counts are elevated in the effusions of patients with TB [25]. This may explain the finding of the present study that TB patients with peritoneal or pleural effusions had significantly lower blood lymphocyte counts than those without effusions (p = 0.035).
These are four limitations in the present study. First, although this is the first study to comprehensively investigate the prevalence of HHV-8 in TB patients with peritoneal or pleural effusions, all participants were recruited from a single teaching hospital. Thus, the results may not be representative of the overall situation in Taiwan or elsewhere in the world. Second, the PCR method used in this study can only detect 5–10 copies of HHV-8 DNA, and therefore may be insufficiently sensitive for definitively establishing the presence or absence of HHV-8 DNA. The question of whether patients such as those who participated in this study have HHV-8 DNA at levels less than 5–10 copies/mL be resolved if the detection sensitivity can be increased. Third, we did not investigate the potential for a common underlying cause of both TB and HHV-8 infection, such as socioeconomic status, nutritional conditions, and health-related behaviors. Fourth, we did not have records of any co-morbidities or adjunct therapies in our TB population, and we expect these to be relatively common in older individuals, such as those in our study population.