Erythrocytes from gilthead seabream (Sparus aurata) and European sea bass (Dicentrarchus labrax) did not engage in phagocytosis

Phagocytosis is a fundamental innate immune process primarily executed by vertebrate leucocytes. Fish erythrocytes are involved in immune functions, although their role in phagocytosis remains poorly investigated. Consequently, this study aimed to examine the potential phagocytic mechanisms of fish erythrocytes. To this end, systemic blood erythrocytes were isolated from the teleost gilthead seabream (Sparus aurata) and European sea bass (Dicentrarchus labrax) and incubated for various times (20, 40, 60, 80, 100, and 120 min) with formalin-inactivated bacteria or heat-killed yeasts labelled with fluorescein isothiocyanate. Their putative phagocytic properties and respiratory burst activity were investigated. Our results did not indicate variations in the phagocytic ability or phagocytic capacity of erythrocytes of seabream or sea bass incubated with the bacteria, whereas no activity was detected in the case of incubation with yeast. Additionally, no respiratory burst activity was detected in the erythrocytes of either fish species under any of the experimental conditions tested. Using fluorescence microscopy, it was observed that erythrocytes could bind bacteria to their surface membranes. However, this attachment process was rarely seen with yeast cells. In contrast, examination of the fine structure of erythrocytes using transmission electron microscopy showed distinctive inward and outward folding (pseudopodia formation) and some cytoplasmic vesicles in both species. The results of our study indicate that erythrocytes from gilthead seabream and sea bass did not exhibit phagocytic capabilities when exposed to these specific target particles. Additional studies are necessary to gain more insights into how they contribute to the immune system of fish.