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In jawed vertebrates, B cell receptors (BCR) are primary pathogen detectors that activate downstream signaling pathways to express adaptive immune effectors. In jawless vertebrates, the variable lymphocyte receptors (VLR) B positive lymphocytes can express and secrete specific VLRB molecules in an analogous manner to that of immunoglobulins by B cells in jawed vertebrates. Our study is the first to demonstrate the possibility of incubation of fertilized eggs and artificial breeding of Lampetra morii larvae throughout their life cycle under laboratory condition. We also found that VLRB, lamprey B-cell linker (L-BLNK), and lamprey nuclear factor-kappa B (L-NF-κB) play key roles in early larval development. Aeromonas hydrophila was found to be a lethal pathogen of L. morii larvae causing rapid infection at a concentration of 107 cfu/mL qRT-PCR results revealed that gene expression levels of VLRB, L-BLNK, and L-NF-κB were up-regulated significantly. Ten-day infection trials showed that VLRB, L-BLNK, and L-NF-κB are crucial for lamprey immune response. Furthermore, the expression levels of L-BLNK and L-NF-κB were down-regulated drastically both at mRNA and protein levels after bacterial infection than in the naive group of VLRB morphants. A similar expression pattern of VLRB and L-BLNK was found in L-NF-κB morphants post bacterial infection. The results were strikingly different in the other two morphants. The VLRB and L-NF-κB expression levels were found to be down-regulated at mRNA and protein levels by less than 30% and 45%, respectively, in L-BLNK morphants compared to those in the naive group. These results indicate that L-BLNK and L-NF-κB might participate in VLRB-mediated immune response. Additionally, in VLRB morphants, the mRNA expression levels of some genes, especially the ones expressed in VLRB+ lymphocytes but not in VLRA+ lymphocytes, were found to be affected. Therefore, these findings of B-like lymphocytes in lamprey offer key evidence with regard to the evolution of adaptive immunity. • We report that L. morii has been cultured in laboratory through the complete life cycle, and VLRB, L-BLNK and L-NF-κB have been proven to be critical for its early development. • We provide evidences that L-BLNK and L-NF-κB seem to be key signaling molecules in VLRB signaling pathway in immune responses of VLRB+ lymphocytes. • These findings of B-like lymphocytes in lampreys offers key evidences into the evolution of adaptive immunity. [ABSTRACT FROM AUTHOR]

Abstract: Nuclear factor of kappa B (NF-κB) is a stimuli-activated transcription factor, regulates the expression of a diverse array of genes. Inhibitor of kappa B-epsilon (IκB-ɛ) is an inhibitor of NF-κB, which retains NF-κB in an inactive state in the cytoplasm. Lampreys (Lampetra japonica) belong to the lowest class of vertebrates with little information about its IκBs. We have identified a cDNA sequence IκB-ɛ-like in the lamprey and the deduced amino acid sequence of IκB-ɛ-like. It contains a conserved DSGxxS motif and six consecutive ankyrin repeats, which are necessary for signal-induced degradation of the molecule. Phylogenetic analysis indicated it had high sequence homology with IκB-ɛs from other vertebrates. FACS analysis showed that IκB-ɛ-like located in cytoplasm of leukocytes. The degradation of IκB-ɛ-like could be observed in leukocytes of L. japonica stimulated with lipopolysaccharide. These results indicate that IκB-ɛ proteins are conserved across vertebrates and the NF-κB-like signaling pathway may exist in the oldest agnatha. [Copyright &y& Elsevier]