Changes in the plasma proteome of Manduca sexta larvae in relation to the transcriptome variations after an immune challenge provide evidence for a high molecular weight immune complex

Manduca sexta is a lepidopteran model widely used to study insect physiological processes including innate immunity. In this study, we explored the proteomes of cell-free hemolymph from larvae injected with a sterile buffer (C) or a mixture of bacteria (I). Of the 654 proteins identified, 70 showed 1.67 to >200 fold abundance increases after the immune challenge; 51 decreased to 0−60% of the control levels. While there was no strong parallel between plasma protein levels and their transcript levels in hemocytes or fat body, the mRNA level changes (i.e. I/C ratios of normalized read numbers) in the tissues concurred with their protein level changes (i.e. I/C ratios of normalized spectral counts) with correlation coefficients of 0.44 and 0.57, respectively. Better correlations support that fat body contributes a more significant portion of the plasma proteins involved in various aspects of innate immunity. Consistently, ratios of mRNA and protein levels were better correlated for immunity-related proteins than unrelated ones. There is a set of proteins whose apparent molecular masses differ considerably from the calculated M_r’s, suggestive of post-translational modifications. In addition, some low M_r proteins were detected in the range of 80 to >300 kDa on a reducing SDS-polyacrylamide gel, indicating the existence of high M_r covalent complexes. We identified 30 serine proteases and their homologs, eleven of which are known members of an extracellular immune signaling network. Along with our quantitative transcriptome data, the protein identification, inducibility, and association provide leads toward a focused exploration of humoral immunity in M. sexta.