Coexpression of molecular chaperone BiP improves immunoglobulin solubility and IgG secretion from Trichoplusia ni insect cells

Infection of Trichoplusia ni (BTI-TN5B1-4) insect cells with a baculovirus coding for immunoglobulin G resulted in significant intracellular insolubility of the immunoglobulin chains. In order to increase the Immunoglobulin solubility, the chaperone BiP was coexpressed in the insect cells using a separate baculovirus vector. This heterologous BiP was observed to associate with immunoglobulin chains in vivo and enhance the level of soluble intracellular and secreted IgG obtained from T. ni. Pulse chase studies indicated that the heterologous BiP increased the level of soluble nascent immunoglobulin chains and assembly intermediates to suggest that BiP is acting as a true molecular chaperone. The effect of heterologous BiP became more significant with time post-infection as secreted IgG levels increased by 90% after 3.4 days of baculovirus infection. Even following the treatment of cells with tunicamycin, BiP coexpression still enhanced immunoglobulin solubility and secretion to indicate that BiP does not function specifically to retain unglycosylated proteins in the endoplasmic reticulum. Thus, coexpression of a molecular chaperone may be used to enhance cellular productivity and protein secretion provided that the chaperone is involved with post-translational processing and significant protein aggregation is observed.