Seroprevalence has the potential to be useful for decision-making at multiple stages of trachoma control programs, from mapping to stopping MDA to long-term monitoring and evaluation. We are focusing on serological testing in terms of making decisions to cease MDA and long-term surveillance, since recent funding from the Department for International Development (DFID) in the United Kingdom to map over 1200 districts in over 30 countries suspected to be trachoma-endemic (http://www.sightsavers.org) makes the validation of serological tools for mapping a low priority. The program end-point is not the interruption of trachoma transmission, but the elimination of blinding trachoma as a public health problem . Recrudescence of the disease long-term is still possible given the right conditions; therefore long-term monitoring strategies that can be easily integrated into ongoing national surveillance systems for disease and/or vaccine coverage offer the best option for surveillance once the currently defined end-points have been met and elimination programs have ceased.
In previous studies , IgG responses to C. trachomatis antigens were measured in children following multiple rounds of MDA. IgG responses correlated with ocular pathology and infection status, but also increased with age, making it difficult to determine if serological responses were due to an active or previous infection or to assess the impact of multiple rounds of MDA. The study reported here provides a first look at antibody responses in a high prevalence community before MDA. Baseline analysis showed good correlation between IgG responses, increasing TF/TI score, and PCR-positive specimens. In areas of high TF prevalence, IgG serology shows that well over 1/3 of children age 3 and under have been exposed to trachoma antigens, while almost 100% of children age 6 have been exposed. Analysis of IgG responses 6 months after MDA showed a significant decrease in antibody levels (represented here by MFI) in all age groups, but no instances of seroreversion, within 6 months following treatment. The lack of a significant decline in IgG responses is likely due to the presence of long-lived memory B cells or plasma cells induced by multiple exposures to ocular chlamydia experienced by children in endemic regions . One could hypothesize from the data that the steeper decline in IgG responses after treatment in younger children is related to a lower overall antigen load experienced as a result of fewer exposures, and that an increase in the number of exposures over time repeatedly boosts memory B cells and plasma cells to become long-lived. Data from murine models [15, 16], vaccination for smallpox [17–19] and other viral infections , and examination of humoral responses from 1918 influenza pandemic survivors nine decades later  have all shown that plasma cells can be incredibly long-lived, being present decades after the initial stimuli. However, it is possible that the local rather than systemic nature of ocular chlamydia infection requires multiple exposures to generate long-lived plasma cells to maintain high levels of circulating antibody.
IgA responses were also examined to determine if mucosal responses may be a better indicator of recent infection, as reported for Campylobacter infection [22, 23]. Fewer children in this study had IgA responses than IgG responses, although this difference was more pronounced in older age groups. Virtually the same percentage of 1–3 year olds had IgG and IgA responses (39.4 vs. 36.1%, respectively), while fewer 4–6 year olds had IgA responses (86.4% IgG + compared to 64.1% IgA+). The MFI for positive IgA samples was overall about 1–1.5 log fold lower than for positive IgG responses, which is to be expected based on the overall lower IgA titers in serum than IgG [24, 25]. IgA responses are related to the total antigen dose, whether a large bolus or repeated infections , so the lower percentage of children seroprevalent for IgA may represent a lower antigen load and failure to seroconvert to IgA positivity. Following drug treatment, IgA levels decreased at each age group more so than the corresponding IgG responses. In children between the ages of two and five, the decrease in IgA responses to CT694 were not as significant as other age groups, and low samples sizes may have contributed to the variance in slopes for these groups. The large variance in responses, in conjunction with a low sample size, does not produce an ideal normal distribution to compare the mean antibody responses from baseline to six months post treatment (data of remaining slopes not shown). While IgA responses can be long-lived, IgA levels will decline following exposure to other mucosal organisms , which may also contribute to the low IgA titers, seroprevalence, and more significant post-treatment decline than IgG.
The use of IgA analysis in a programmatic context poses some problems. IgA is the most prevalent isotype in mucosal surfaces but is a minor antibody isotype in the blood, which will be the sample type almost certainly used for large scale monitoring and evaluation of trachoma control programs. From a technical standpoint, testing for IgA poses additional problems for high through-put surveillance of large populations. IgG depletions prior to IgA analysis is recommended due to the lower serum concentrations of IgA than IgG [24, 25] and this additional step adds significantly to time and costs. Testing for IgA also negates the ability to multiplex with IgG responses of other disease antigens. IgM may be a better option for picking up recent infections, but the same obstacles to using this in a high-throughput, integrated platform as IgA remain. From a technical standpoint, IgA assays in this study had higher CVs than IgG assays did, with almost a quarter of the samples having CVs of greater than 15% for one or both antigen(s). This is likely due to the low concentration of IgA in the serum  and possibly through the binding of the more abundant IgG to antigen, sterically hindering the binding of IgA in the assay. This unexpected technical difficulty represents an additional obstacle to using IgA serology on a broad platform for monitoring and evaluating trachoma programs.