Hemoglobin mediates inflammation and apoptosis in the head-kidney macrophages of grass carp (Ctenopharyngodon idella)

Hemorrhagic venereal infection or tissue injury usually results in the hemolysis of red blood cells (RBCs) and releases the endogenous damage-associated molecular pattern hemoglobin (Hb) into the plasma or tissues. The redox-reactive Hb induces oxidative stress and inflammation, disrupting the redox balance and impairing the immune response of immune cells. However, how the cytotoxic Hb affects the head-kidney macrophages in grass carp remains unclear. Here, we showed that the head-kidney macrophages in grass carp were capable of engulfing large amounts of Hb. The incubation with Hb increased the level of intracellular reactive oxygen species (ROS) in head-kidney macrophages through the Fenton reaction of Hb. The results of quantitative real-time PCR and RNA-seq analyses indicated that the high oxidation activity of Hb up-regulated the expression of proinflammation cytokines in head-kidney macrophages. Apoptosis detection suggested that the incubation of Hb induced the apoptosis of macrophages and up-regulated the transcription of apoptosis-related genes, which was likely mediated through the ROS released by Hb. Overall, these data demonstrate that Hb can stimulate inflammation and induce apoptosis in head-kidney macrophages.