PADI4 and COVID-19: Thus, as PAD4 is mainly expressed by hematopoietic cells, with the highest levels in neutrophils [34], the absence of a difference suggests that the global citrullination activity is not different between the critical COVID-19 and septic shock groups, supporting the assumption that a part of the circulating Nu.H3.1 should originate from cell types other than neutrophils.