Figure 6.

Proposed mechanism of heavy-chain antibody induction by genetic immunization. The figure illustrates the likely mechanism of heavy-chain antibody induction by genetic immunization of a camelid. (A) Ballistic injection of gold particles coated with cDNA expression vectors (rings) transfects some epithelial cells (EC) and antigen-presenting cells (APC) in the skin. Injury of the plasma membrane by the penetrating gold particles results in the release of danger-associated molecular patterns (DAMPs). Here, gold particles carry a cocktail of expression vectors encoding a mouse G-protein-coupled receptor (GPCR) (red) and its human homolog (blue) as a fusion protein to green fluorescent protein (green). Transcription and translation results in cell surface expression of native mouse and human GPCRs. Peptides derived from these proteins are presented by MHC-I molecules on the cell surface. A cytotoxic T cell (Tc) with a matching T cell receptor (TCR) is induced to secrete perforins. Insertion of perforins into the plasma membrane of the EC/APC results in lysis of the target cell. (B) Exosomes and cellular debris released during cell lysis are transported to the draining lymph nodes. B cells with a matching heavy-chain B cell receptor (BCR) endocytose the immunogen-containing exosomes and debris. Peptides derived from endocytosed proteins are presented on the B cell surface by MHC-II molecules. A helper T cell (Th) with a matching TCR releases growth-promoting cytokines. Follicular dendritic cells (FDCs) bind immune complexes via the low-affinity Fc receptor (FcγRIIb). FDCs present complexes of antibodies and immunogen to B cells expressing matching heavy-chain antibodies. Th cytokines and survival factors provided by FDC induce: (i) clonal expansion of B cells expressing specific heavy-chain antibodies, (ii) somatic hypermutation of amino acids in the heavy-chain antibodies (red asterisks), (iii) selection of B cells expressing higher affinity antibodies, and (iv) differentiation of specific B cells into antibody-secreting plasma cells (PC). The latter emigrate from the lymph node and migrate via the vascular system to the bone marrow, where some of these cells settle in long-lasting niches. (C) Explanation of the logos used in the diagrams to depict cell surface proteins.