Recombinant Baculoviral Production of Class I Major Histocompatibility Complex Tetramers

Identifying immunoreactive cytotoxic T lymphocytes (CTLs) by current technologies (cytokine secretion, intracellular cytokine, ELISPOT, and MHC tetramer assays) is often difficult when probing for multiple target antigens. CTLs activate and induce apoptosis of pathogenic cells when T-cell receptors (TCRs) specifically bind to antigenic peptides and major histocompatibility complexes (pMHCs) presented on the target cell’s surface. Flow cytometric MHC class I tetramer assays allow for the direct quantification and sorting of most CD8+ T lymphocytes whose TCRs recognize bound peptides, regardless of effector function. Class I tetramers are traditionally produced using BL21-DE3 E. coli expression, denaturation and folding in vitro, which is technically challenging, time-consuming, and low-throughput. We are developing an assay amenable to rapid, high-throughput screening of peptide libraries to characterize and quantitate antigen-specific CTLs in peripheral blood mononuclear cells (PBMCs). Baculovirus expression systems, utilizing host eukaryotic chaperones and isomerases, are capable of producing soluble, properly-folded protein complexes with high yields. The HLA-A*0201 heavy chain and beta-2-microglobulin genes were cloned into pIEx baculovirus expression vectors. Recombinant HLA-A*0201 and β2m viruses were synthesized using the BacMagic-3 DNA/pIEx method and transfected into Spodoptera frugiperda (Sf9) cells, and protein expression was confirmed by Western blot. To prepare T cells for testing, PBMCs from a healthy HLA-A2+ donor were collected and pulsed with DMSO control or CEF peptide pool (a mixture of CMV-, EBV-, and Flu-specific HLA class I epitopes). After 5 days, the CD8+ and CD8- fractions were sorted by MACS-based magnetic separation, and the frequency of FluM1-specific lymphocytes in the CD8+ populations was determined (0.1% of DMSO control vs. 0.772% of CEF-pulsed cells) using a commercial tetramer. We are optimizing HLA-A*0201 and β2m baculovirus co-infection ratios and evaluating the efficiency of intracellular MHC folding.