Abstract
To seek evidence of a primitive adaptive immune system (AIS) before vertebrate, we examined whether lymphocytes or lymphocyte-like cells and the related molecules participating in the lymphocyte function existed in amphioxus. Anatomical analysis by electron microscopy revealed the presence of lymphocyte-like cells in gills, and these cells underwent morphological changes in response to microbial pathogens that are reminiscent of those of mammalian lymphocytes executing immune response to microbial challenge. In addition, a systematic comparative analysis of our cDNA database of amphioxus identified a large number of genes whose vertebrate counterparts are involved in lymphocyte function. Among these genes, several genes were found to be expressed in the vicinity of the lymphocyte-like cells by in situ hybridization and up-regulated after exposure to microbial pathogens. Our findings in the amphioxus indicate the twilight for the emergency of AIS before the invertebrate-vertebrate transition during evolution.
Highlights
The emergence of adaptive immunity represents a major step in the host-pathogen arm race that has led to the current highly elaborate immune response system in vertebrates
The immune system is functionally compartmentalized into primary lymphoid organs and secondary lymphoid tissues in vertebrates
The primary lymphoid tissues are the place where lymphocyte precursors develop into immunocompetent naive lymphocytes
Summary
The emergence of adaptive immunity represents a major step in the host-pathogen arm race that has led to the current highly elaborate immune response system in vertebrates. The identification of lymphocyte-like cells and molecules participating in the immune response and recognition processes in lamprey [1], the oldest living jawed vertebrate, suggests the emergency of adaptive immune system (AIS) before or shortly after the dawn of vertebrate evolution approximately 500 mya ago. It remains unclear exactly when AIS first appeared during evolution. The findings of lymphocyte-like cells and their related genes in amphioxus represent initial appearance of AIS before the invertebratevertebrate transition during evolution
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