Aluminum as vaccine adjuvant increases immunity, but deplete splenic melanomacrophage centers in sea bream

Abstract

Aluminum as vaccine adjuvant has been proved to be successful in several species at inducing increased immune responses against the many poorly immunogenic antigens. Fish are not the exception and promising results with different antigens in many species are on-demand. Even though, the mechanism of action of this adjuvant often remains unclear. In this study, we tested by IP a commercial formulation of aluminum hydroxide and magnesium (Imject Alum) as adjuvant of the model antigen keyhole limpet hemocyanin (KLH), and found that it has as a potent and unique adjuvancy capacity. Compared to control group, two commercial preparations (Montanide 761 or 763) or KLH alone, Imject Alum plus KLH formula, despite the powerful immunogenic activity recorded over the innate (IL-1b expression and serum lysozyme)and adaptive (total serum and skin mucus IgM) immune responses throughout the 8 months of trial, it also caused several damages at the injection site. Among damages, severe adhesions, granulomatosis and a transitory melanization were recorded, but, without obvious signs of impaired animal health. Additionally, optical screening for external tissue damage was performed, but any change was observed. Surprisingly, histopathology evaluation trough classical stains of head-kidney, spleen, liver and gut revealed that melanomacrophage centers at the spleen were depleted of cells, but not in HK. Furthermore TEM confirmed this data which suggest iron retention in MMCs or a high catabolic activity exerted by erythrocytes as possible mechanisms associated with depletion. Whatever the case, the use of aluminum as vaccine adjuvant must be further explored in detail before the commercial application in aquaculture facilities.