Parasitology and immunology of mice vaccinated with irradiated Litomosoides sigmodontis larvae

This study was performed with Litomosoides sigmodontis, the only filarial s pecies which can develop from the infective larvae to the patent phase in i mmunocompetent laboratory BALB/c mice. Parasitological features and immune responses were analysed up to 3 months before and after challenge inoculati on, by comparing 4 groups of mice: vaccinated challenged, challenged only, vaccinated only, and naive mice. Male larvae were very susceptible to irrad iation and only female irradiated larvae survived in vivo. Protection, asse ssed by a lower recovery rate, was confirmed and was established within the first 2 days of challenge. This early reduction of the recovery rate in va ccinated challenged mice was determined by their immune status prior to the challenge inoculation. This was characterized by high specific IgM and IgG subclass (IgG1, IgG2a and IgG3) levels, high specific IL-5 secretion from spleen cells in vitro and a high density of eosinophils in the subcutaneous connective tissue. Six h after the challenge inoculation, most tissue eosi nophils were degranulated in vaccinated challenged mice. Thus, in the proto col of vaccination described, protection appeared mainly to result from the stimulation of a Th2 type response and eosinophils seemed to be the main e ffecters for the increased killing of infective larvae in vaccinated challe nged mice. Two months after challenge inoculation, the percentage of microf ilaraemic mice was lower in vaccinated challenged mice as a consequence of this overall reduction in the worm load. In both vaccinated challenged and challenged only groups, the in vitro splenocyte proliferative capacity was reduced in microfilaraemic mice.