Reply to “Chronic Vaginal Candidiasis Is Achievable in Outbred CD-1 Mice”

We appreciate the attention given to our publication regarding the mechanism associated with the immunopathogenic response in experimental vulvovaginal candidiasis that employed mice resistant (CD-1) and susceptible (C3H/HeN) to chronic colonization/infection (6). We respect data presented by Gabrielli et al. (1) regarding the achievement of a chronic colonization condition in the CD-1 strain of mice that were characterized as resistant in our report (6). As an experimental model system, the conditions can often be pushed in one direction or another that may produce variable phenotypes. We believe that these contrary observations are a classic example of employing conditions that altered the phenotype. Our laboratory published data for the vaginal infection using inocula of 5 104 to 5 105 blastoconidia and exogenous estrogen between 0.2 and 0.5 mg/mouse/week for several years based on early optimization studies. We then conducted a lower-limit analysis for experimental vaginitis using mice susceptible to long-term colonization that included a dose response of inocula (1 102 to 5 104 blastoconidia) as well as exogenous estrogen (0.002 to 0.2 mg/week) (2). On the basis of these studies, 5 104 blastoconidia were found to be an optimal inoculum based on the fact that lower inocula (1 102 to 1 103) resulted in variability in fungal burden. In addition, the 5 104 inoculum reproducibly resulted in stable fungal burden at levels similar to those observed at higher inocula (1 106 to 1 107). The estrogen dose was chosen in a similar manner (0.02 mg/week). Interestingly, the estrogen dose was ultimately increased to 0.1 mg/week, as random variability in fungal burden was observed when different lots of estrogen were used. The inoculum used by Gabrielli et al. (1) was 40 higher (2 106) than the inoculum we optimized for the susceptible mice and even higher (400 at 2 107) in their previous published work (3). Therefore, it is not surprising that the resistant phenotype was overcome, resulting in a conversion toward a susceptible phenotype. To examine the inoculum issue further, we conducted a study that evaluated the effect of inocula in susceptible (C3H/HeN) mice on the outcome of the acute inflammatory response via polymorphonuclear neutrophil (PMN) migration and ultimately, vaginal fungal burden. Our goal was to determine whether the susceptible phenotype could be similarly converted to a resistant phenotype with lower inocula. For this, the standard inoculum (5 104), lower-limit inoculum (1 102), and low inoculum (5 102) of Candida albicans 3153A was used. Note that the lower-limit and low inocula were 500-fold and 100-fold, respectively, below the standard inocula. Longitudinal analysis of PMN migration and vaginal fungal burden was monitored for 15 days. Results showed that mice receiving the lower-limit inoculum did not become colonized and did not show PMN migration to the vaginal cavity. Hence, the lower-limit inoculum in the susceptible mice did not show a resistant phenotype with a clearing PMN response. Instead, it appeared that the inoculum was too low to support colonization to trigger a PMN response. Mice receiving the low and standard inocula both showed Published 24 October 2017 Citation Yano J, Noverr MC, Fidel PL, Jr. 2017. Reply to “Chronic vaginal candidiasis is achievable in outbred CD-1 mice.” mBio 8: e01736-17. https://doi.org/10.1128/mBio .01736-17. Editor Liise-anne Pirofski, Albert Einstein College of Medicine Copyright © 2017 Yano et al. This is an openaccess article distributed under the terms of the Creative Commons Attribution 4.0 International license. Address correspondence to Paul L. Fidel, Jr. [email protected]. This is a response to a letter by Gabrielli et al. (https://doi.org/10.1128/mBio.01372-17). AUTHOR REPLY