Formins (FHOS) in CD21-mediated signaling & virus entry

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DESCRIPTION (provided by applicant): Human CD21 is an innate immune system receptor (C3dg receptor or complement receptor type 2, CR2) and is a key modulator of the acquired immune response. CD21 is also the high affinity receptor for EBV. CD21 plays a direct role in the pathogenesis of a broad spectrum of human disease, including autoimmune disease, infectious disease (EBV and HIV) and cancer. On B cells, CD21 complexes with the major signal transduction regulator CD19, and it is controversial whether CD21 can signal independently of CD19. Although CD21 was identified more than 20 years ago and distinct roles as an attachment protein for C3dg, CD23 and EBV have been discovered, the precise mechanisms enabling CD21 to directly relay information between the exterior and the interior of the cell are unresolved. In Preliminary Studies, FHOS/FHQD a member of the formin family, was found to bind the cytoplasmic domain of human CD21 both in vitro and in vivo. When expressed in epithelial cells (CD19 negative), EGFP-FHOS localized to the cytoplasm and accumulated with actin in membrane protrusions. Remarkably, when CD21 on epithelial cells was ligated with either EBV or anti-CD21, FHOS translocated to the plasma membrane, where it co-localized in aggregates with CD21. Formins are scaffolding proteins that regulate cell polarity and morphogenesis by linking signal transduction to cytoskeletal rearrangement and gene transcription. Recently, formins were discovered to directly nucleate actin by a pathway distinct from the classic Arp 2,3 complex. Based on these observations it is proposed that ligand stimulation of CD21-FHOS interaction transduces intracellular signals through a pathway directed by formin (FHOSV-mediated cytoskeletal reorganization). The goals of Aim I are to identify the amino acid residues that mediate interaction between CD21 and FHOS and to screen for novel FHOS/CD21-binding proteins as potential downstream effectors of this pathway. Aim II is focused on determining whether FHOS directly nucleates actin, and assessing the role of CD21 in FHOS activation, actin polymerization, and signaling. The goals of Aim III are to isolate and characterize the splenic littoral cell, a specialized cell abundant in human red pulp that highly expresses FHOS, CD21 and is believed to function in antigen filtration. Achievement of these Aims will provide important new knowledge about how CD21 signals and will elucidate the role(s) of the recently discovered formin protein, FHOS, in the immune system. These investigations may lead to identification of novel targets for treatment of autoimmune disease and cancer, for development of vaccines and for eradication of persistent viral and bacterial pathogens that are sequestered by CD21.