Immune response in BALB/c mice to an attenuated strain of Nocardiabrasiliensis

We recently developed an attenuated strain of Nocardia brasiliensis HUJEG-1, named NBP200, through 200 continuous passaging. When administered to naïve mice, this attenuated strain confers resistance to infection with the wild-type strain. Although the immune response to the parental strain has been previously studied, the analysis of the immune response to the attenuated strain will help to understand the immune mechanisms involved in resistance against this microorganism.

Initially, experimental mice were divided into two groups and infected with either the attenuated or the wild-type strain to compare their responses. After 12 weeks, animals initially inoculated with NBP200 were challenged with the wild-type isolate (NBP0), while a control group of the same age was infected with NBP0. Flow cytometry was employed to analyze surface and intracellular markers, including gamma interferon, IL-4, IL-10, and IL-17, in order to assess Th-cell responses in the spleen and infected footpad. Sera were collected at various time points for ELISAs and Western blot assays in order to characterize the humoral immune response.

Levels of gamma interferon and IL-4 cells were similar in both groups, whether inoculated with the attenuated or wild-type strain. However, concentrations of IL-17 and IL-10 were notably higher in animals inoculated with the wild-type strain, especially at 90 days post-inoculation. In the mice inoculated with the attenuated strain there were not any observable mycetoma lesions, and the quantity of IL-10+ or IL-17+ cells was minimal. In the re-infected animals, the most significant difference was observed in the Treg response during the late stage, where an immunosuppressive microenvironment was not observed. IgG levels in mice infected with either NBP0 or NBP200 were similar. In contrast, those inoculated with NBP200 and challenged with NBP0 exhibited a robust humoral response characterized by antibodies against P61, as well as to other proteins not previously reported, including those with molecular weights of 13.8-, 15.8-, 66-, and 69-kDa.

The attenuated strain and the re-infected group exhibited different modulation of the Th cell response. Immunized animals with NBP200 and challenged with NBP0, showed a rapid and strong IgG antibody response. These changes may be potential factors contributing to the protective immunity against the wild-type strain.