We undertook the present analysis to compare patterns of plasma markers of host immune status in Japanese and Jamaican carriers and non-carriers of HTLV-I using standardized laboratory assays. Some cytokines may be unstable, or present at levels below the lower detectable limits of commercial assays, in the plasma specimens available for our investigation (30
). Thus, we measured soluble protein correlates of cytokine levels that are well accepted as plasma immune markers, including several that we previously measured successfully (32
). We included EBV antibody profiles as markers of cellular immunity; studies have shown that the combination of lower anti-EBNA1 and higher anti-EBNA2 titers suggest poor control of EBV infection in the context of a weak CTL response (22
). We also measured concentrations of plasma markers of T-cell (sIL2R, sCD30), B-cell (total IgE), and inflammation or non-specific (CRP, neopterin) immune activation (7
). Compared to cytokines, the surrogate markers are considered to be more stable, more frequently detectable, and measurable with greater precision in peripheral blood specimens from asymptomatic individuals (30
To characterize underlying population differences in host immunity, we examined immune marker patterns in Japanese and Jamaican non-carriers of HTLV-I. In the Jamaican non-carriers we observed marker patterns that suggest less T-lymphocyte activation (i.e., lower mean anti-EBNA1, sIL2R, and sCD30 levels) (25
), and diminished CTL control of EBV (i.e., a greater prevalence of a low EBNA1:EBNA2 ratio) (22
), compared to Japanese non-carriers. This lower level of T-cell activation may confer a lower risk of T-cell transformation and thus contribute to the relatively low risk of ATL in Jamaica. In contrast, higher mean CRP levels in Jamaican non-carriers compared to Japanese non-carriers are consistent with increased levels of inflammation or non-specific immune activation in the Jamaicans (27
). The Jamaican subjects’ mean CRP levels were lower than those typically associated with inflammation-related disease (34
); however, such subclinical increases in inflammation may contribute to the greater risk of HAM/TSP in Jamaica. The small sample sizes in the present analysis, as well as the use of surrogate rather than direct cytokine markers, warrant cautious interpretation. Nonetheless, the contrasting immune marker patterns in Japanese and Jamaican non-carriers of HTLV-I may reflect underlying population differences in host immune status. These differences may be due, in part, to genetics, nutritional status, social environment, and/or co-morbidity such as parasitic burden.
We also observed marked population differences in HTLV-I-associated immune marker patterns. Diminished cellular immunity, as characterized by a low EBNA1:EBNA2 ratio (22
), was more frequent in Japanese carriers than non-carriers, consistent with previous reports (13
). In contrast, the prevalence of a low EBNA1:EBNA2 ratio did not differ by HTLV-I status in the Jamaican subjects, but was greater in both the carriers and non-carriers from Jamaica than in the Japanese carriers. Thus, the apparent population differences in HTLV-I-related EBV antibody patterns reflect the population differences we observed among the HTLV-I non-carriers.
Similarly, among Japanese subjects, HTLV-I carriers had higher CRP levels than non-carriers, but the median CRP levels were even higher in Jamaican subjects regardless of HTLV-I status. The HTLV-I-related increase in CRP levels in the Japanese subjects may reflect increased IL-6 expression (35
), possibly induced by Tax (36
), while the higher CRP levels in Jamaican non-carriers may have obscured any Tax-induced effects on CRP or IL-6 in the Jamaican carriers.
HTLV-I infection of T-cells results in up-regulation of activation markers, including IL2R and CD30 (25
), on infected cells. In addition, elevated plasma sIL2R (38
) and sCD30 levels (40
) have been reported in ATL patients and likely reflect tumor burden related to IL2R and CD30 expression by ATL cells (25
). HAM/TSP patients and asymptomatic HTLV-I carriers were also reported to have significantly higher sIL2R levels than non-carriers in a predominantly Caribbean population (42
). We previously reported similar sCD30 levels in asymptomatic Japanese carriers and non-carriers (16
), but the levels have not been well studied in Jamaican carriers. In the present analysis, we did not observe differences in sIL2R or sCD30 levels by HTLV-I status in Japanese subjects, whereas levels of both markers were higher in carriers than non-carriers from Jamaica, especially sIL2R. Those apparent HTLV-I-associated differences in sIL2R levels in the Jamaicans were due to lower levels in the Jamaican non-carriers rather than to unusually high levels in the Jamaican carriers. Therefore, the HTLV-I-related increases in these markers among the Jamaicans appear to reflect the lower background population levels of T-cell activation that we observed in the Jamaican non-carriers.
One Jamaican study has previously examined circulating neopterin levels in asymptomatic adults by HTLV-I serostatus (43
) and found that the levels did not vary by HTLV-I status, consistent with the present findings. We also did not observe significant differences in total IgE levels by HTLV-I serostatus in either population, in contrast to prior reports of lower total IgE levels in HTLV-I carriers than non-carriers (16
In analyses restricted to carriers of HTLV-I, the immune markers generally showed more correlation with viral markers in Jamaican than in Japanese carriers. In the Japanese carriers, EBV antibody patterns consistent with diminished cellular immunity had suggestive associations with anti-Tax seropositivity. We conducted a cross-sectional analysis and therefore cannot determine the temporal relation of the viral and immune marker patterns. However, the observed correlations in Japanese carriers may reflect an increase in viral protein expression and division of HTLV-I-infected T-lymphocytes in persons with diminished type 1 immunity (8
). In Jamaican carriers, plasma markers of T-cell (sIL2R, sCD30) (25
) and non-specific (neopterin) (28
) immune activation were correlated with HTLV-I provirus load and anti-HTLV-I titer and had suggestive associations with anti-Tax seropositivity. Those correlations are consistent with the prediction that active host immune responses to HTLV-I in the context of persistent HTLV-I replication characterize the Jamaican carriers in this study population (15
A synthesis of the immune and viral marker data suggests several hypotheses regarding population differences in host immune status and control of HTLV-I. Among Japanese participants, non-carriers of HTLV-I appeared to have increased T-cell activation, but HTLV-I was not associated with a further increase in T-cell activation marker levels. Concurrently, the HTLV-I viral marker patterns in Japanese carriers did not indicate persistent HTLV-I replication or CTL responses to HTLV-I (15
). Thus, the Japanese carriers may not have effective virus-specific immune responses despite the evidence of T-cell activation. This suggestion is consistent with the present and previous observations of diminished type 1 immunity in Japanese carriers (13
). Furthermore, in the absence of evidence for persistent HTLV-I replication, the high provirus loads we observed in Japanese carriers are most likely a result of mitotic division of HTLV-I-infected T-lymphocytes (15
). The combination of ongoing but ineffective T-cell activation and mitotic proliferation of HTLV-I-infected T-lymphocytes could contribute to the increased risk of ATL that is characteristic of Japanese carriers. In contrast, in Jamaican carriers and non-carriers of HTLV-I we observed plasma marker levels that imply chronic inflammation. In addition, Jamaican carriers had plasma T-cell activation marker levels similar to those of Japanese subjects. HTLV-I viral marker levels in Jamaican carriers further indicated persistent HTLV-I replication and virus-specific CTL responses. The combination of persistent HTLV-I replication, CTL responses, and inflammation is characteristic of a population at greater risk for HAM/TSP (11
). The lysis of HTLV-I-infected T-cells by a persistent CTL response may also contribute to the comparatively low risk of ATL observed in Jamaica.
The matched study design, direct comparisons of immune marker profiles across populations, and standardized laboratory assays represent unique strengths of the present analysis. Nonetheless, limitations of this study should be noted. The immune marker findings may not be directly applicable to ATL or HAM/TSP patients, as all of the HTLV-I-infected participants were asymptomatic. Also, we could not adjust for factors that may modulate the host immune response to HTLV-I, including age at infection, route and dose of infection, nutritional status, social environment, co-morbidity, and host genetics. The statistical power to detect significant small associations was limited due to the small sample size. However, we included as many participants as could be appropriately matched across populations; we are not aware of ongoing studies that could yield a larger sample size for a similar analysis.
In conclusion, we observed several intriguing new findings related to population differences in host immune status in carriers and non-carriers of HTLV-I. The findings corroborate the view that host immune dysregulation, perhaps even at a modest, subclinical level, contributes to the well established differences in natural history of HTLV-I infection in Japanese and Jamaican carriers. It remains unclear why HTLV-I infection results in contrasting types of immune dysregulation in the two populations. Possible mechanism(s) may be elucidated by longitudinal studies that include ATL and HAM/TSP patients and information on potential co-factors that was not available for the present analysis. The further development of informative, easily measured markers of immune status will also aid future studies. Those studies will provide important information to characterize the asymptomatic carriers of HTLV-I who are at an increased risk of ATL or HAM/TSP.