Mycopathologia 152: 69–79, 2000. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 69 Cell-mediated immune responses (CMIR) to Rhinosporidium seeberi in mice Sharmini Jayasekera 1 , S. N. Arseculeratne 2 , D. N. Atapattu 2 , R. Kumarasiri 3 & W. M. Tilakaratne 4 1 Animal Centre, Medical Research Institute, Colombo 8, Sri Lanka; 2 Department of Microbiology, 3 Department of Community Medicine, 4 Department of Oral Pathology, University of Peradeniya, Sri Lanka Received 16 January 2001; accepted 2 August 2001 Abstract There is no published data on Cell Mediated Immune Responses in experimental animals to Rhinosporidium see- beri the causative agent of human and animal rhinosporidiosis. The quantitative mouse foot-pad model was used to assay the Delayed-Type Hypersensitivity (DTH) cell-mediated immune response to extracts of purified endospores and sporangia of R. seeberi. Histological examination was used to confirm that the foot-pad reactions were compat- ible with DTH reactions in the mouse. We report that sonically disintegrated rhinosporidial endospores/sporangia induced DTH responses in the foot-pads of sensitized mice which were comparable in intensity and histological profile to that induced by sheep red blood cells in SRBC sensitized mice. Anti-rhinosporidial antibody was also induced. Filtrates of the soluble antigens in sonicated suspensions failed to evoke a DTH-foot-pad (DTH-FP) response in sensitized mice although an anti-rhinosporidial antibody response to this preparation was detected. Prolonged pre-treatment with sonicated suspensions of endospores and sporangia resulted in a decrease of DTH reactivity as compared with reactions following pre-treatment of a shorter duration. Key words: cell-mediated immunity, mouse foot-pad, Rhinosporidium seeberi Introduction Studies on anti-rhinosporidial CMIR will be of im- portance in attempting to understand why rhinospor- idiosis, known for over a hundred years, is chronic, recurs after surgical extirpation of the rhinosporidial polyps and why it occasionally disseminates. There is only one report [1] on cell-mediated immune re- sponses (CMIR), investigated by the Leucocyte Mi- gration Inhibition (LMI) test, in human rhinosporidi- osis. These authors reported that the LMI responses which were detected in cases of under 9 years dur- ation, decreased in older cases; no explanation was given for this decrease. Earlier work which used crude preparations of rhinosporidial bodies in human skin tests gave results which were thought to have been due to non-specific reactions [2]. We have earlier presented findings (discussed be- low) on CMIR in human rhinosporidiosis [3] obtained through immuno-histology on human rhinosporidial tissues, and in vitro lympho-proliferative responses of peripheral blood lymphocytes of rhinosporidial pa- tients to the T-cell mitogen Concanavalin A (Con A) and to extracts of purified rhinosporidial endospores and sporangia. Both sets of data showed that a CMIR does develop in human rhinosporidiosis. No report exists on CMIR in experimental anim- als evoked by Rhinosporidium seeberi, the causative agent of human and animal rhinosporidiosis. This report deals with CMIR induced by rhinosporidial en- dospores and sporangia as assayed by the standardized Delayed-Type Hypersensitivity response in the foot- pad (DTH-FP) of the mouse. Mice have not been reported as natural hosts of R. seeberi, nor indeed has any experimental animal, including the mouse, been shown to reproduce experimental rhinosporidi- osis. The mouse foot-pad DTH reaction, however, has been shown to be a useful model for the investigation of CMI responses to chemical, microbial and mam-