Why are mice lymphocytic and primates granulocytic? an evo-devo approach to granulopoiesis

Abstract Neutrophils play a critical role as first-line, non-specific responders in the innate immune system. They are characterized by their short-lived nature and composition of cytotoxic granules and other enzymes which can be released to kill pathogens as part of the innate immune system. Lymphocytes, on the other hand, are considered a central responder for the adaptive, or specific immune system. These white blood cells will typically respond to specific antigens and protect the host from viral infection. This specific response is therefore considered slower than an innate response, but it results in the generation of immunological memory. Neutrophils are the most abundant type of leukocyte in humans, comprising between 50% - 70% of circulating leukocytes. In mice, however, lymphocytes are predominant (50-75%). Granulopoiesis is controlled primarily by granulocyte colony stimulating factor (G-CSF, also known as CSF3). The receptor protein for G-CSF is encoded by the gene CSF3R. This receptor is essential to the regulation of neutrophil cell differentiation and proliferation during hematopoiesis. Alternative splicing of CSFR3 gives rise to multiple isoforms of the receptor. Three specific isoforms have been described in humans: CSF3R-class 1, CSF3R-class 3, and CSF3R-class 4. Mice lack Csf3r-class 3 and class 4 isoforms. We hypothesize that the switch from lymphopoiesis to granulopoiesis is due to the evolutionary emergence of CSF3R isoforms. First, we performed a literature search and comparative analysis of normal complete blood count values among selected vertebrates. Secondly, we performed bioinformatic analysis of CSF3R using genomic DNA sequences deposited in ENSEMBL to construct phylogenetic relationships among vertebrates. We identified exon/intron boundaries and sequence homologies that would facilitate alternate splicing to produce different CSF3Risoforms. A detailed analysis of the distal part of the gene revealed the existence of cryptic splicing sites in primates. This sequence variation may be a reason for emergence of splicing variants which are not found in other vertebrates, an intermediate splicing variant in chimpanzee and a Class IV in human. Next, using PCR primer sets designed to detect different CSF3R class receptors, we confirmed the absence of Csf3r isoforms 3 and 4 in murine myeloid cell lines and primary hematopoietic tissue. Furthermore, we confirmed the presence of class IV in peripheral blood and bone marrow harvested from rhesus macaque. In summary, we have shown that expression of CSF3R isoforms vary in vertebrate evolution with a skewing toward neutrophils in primates and away from lymphocytes in rodents. We hypothesize that pathogens have shaped divergence of innate and adaptive immune systems. One side effect of a shift toward granulopoiesis may be the collateral damage of chronic inflammatory/autoimmune diseases and cancer.