Fish red blood cells express immune genes and responses

The erythrocytes or red blood cells (RBCs) and leucocytes or white blood cells (WBCs) are the most abundant cell types in the circulatory system of vertebrates. The RBCs are responsible for gas exchange while WBCs are solely involved with immunity. In this study, we present evidence that fish RBCs despite their respiratory role they also express immune-related genes and responses in the teleost fish, Nile tilapia (Oreochromis niloticus). Fluorescent staining identified the presence of numerous mitochondria and lysosomes in tilapia RBCs. RNA sequencing of tilapia RBCs and WBCs led to the assembly and annotation of 20.876 transcripts of which 1017 and 3628 transcripts were specific to RBCs and WBCs, respectively. Pairwise sequence comparisons identified 3251 transcripts that were significantly differential expressed of which 707 (21.75%) were up-regulated and 2544 (78.25%) down-regulated in RBCs when compared to WBCs. 809 transcripts were assigned to 16 immune system pathways, 21 and 163 transcripts were specifically expressed in tilapia RBCs and WBCs respectively, and 23 and 233 transcripts were highly expressed in RBCs and WBCs respectively. Expression of 7 immune genes (interferon regulatory factors (irf) 1-5 and 9 and tlr3) in both WBCs and RBCs was validated by RT-PCR analysis. Quantitative real-time PCR revealed that all genes were up-regulated in poly(I:C)-challenged WBCs and irf (1, 3, 4, and 9) and tlr3 were also significantly up-regulated in poly(I:C)-challenged RBCs. Therefore, fish RBCs expressed immunity genes and responses and they may play a complementary role in vertebrate immunity.