• Characterization of Plasma Cell Lineages in the Nurse Shark

      Castro, Caitlin Doremus; Flajnik, M. F. (Martin F.) (2013)
      IgM was the first immunoglobulin (Ig) isotype to arise in evolution. Cartilaginous fish, including sharks, skates, and rays, are the oldest animals with an Ig-based adaptive immune system. Sharks express a germline-joined isotype, IgM<sub>1gj<sub>, as well as two forms of IgM, the typical pentameric (19S) form, which is attached to J chain, and a monomeric (7S) form devoid of J chain. Preliminary evidence suggests that 19S IgM consists of low affinity antibodies, whereas the 7S monomers can represent high affinity antibodies that function similar to mammalian IgG. Germline-joined IgM<sub>1gj<sub>, and some 19S IgM are secreted early in ontogeny, while adult sharks produce 19S and 7S IgM. The developmental relationship among the B-cells that produce these different isotypes is unknown. In one model, 19S producers "switch" to secrete 7S IgM by silencing J chain expression after activation by antigen and T cells. In another model, there may be two or more completely separate lineages that give rise to early 19S, late 19S and 7S IgM-secreting B-cells. In order to study the developmental relationship between 19S and 7S IgM-producing B cells in the nurse shark, we examined differences in transcription factor expression, Ig gene usage, heavy/light chain pairings, and sequence signatures from neonatal and adult IgM. In these studies we found a population of Blimp1-negative 19S-secreting cells in neonatal and adult nurse sharks, and unique pairing of a particular IgM H with a σ' L chain in neonatal sharks. In addition to 19S IgM, neonatal nurse sharks express the germline-joined isotype, IgM<sub>1gj<sub>. Therefore, we also examined L chain pairings in IgM<sub>1gj<sub>-secreting cells. Our findings suggest that a κ IgL pairs with IgM<sub>1gj<sub>, and that IgM<sub>1gj<sub> binds to self-molecules in neonatal gill and other tissue. Lastly, we attempted to devise a protocol for the separation of 19S- from 7S-secreting cells in order to examine the IgM repertoire in these populations. These studies provide greater insight into the function of early/innate antibodies in sharks, and we predict that they will also be relevant to mammalian models of early antibody secretion.