Identification and Characterization of CMV-Specific, HLA-C*07:02 Restricted Antibodies

In patients with insufficient T-cell responses to the human cytomegalovirus (HCMV), reactivation can occur which significantly contributes to morbidity and mortality after allogeneic stem cell transplantation (HSCT). Despite the event of new therapeutics like letermovir, HCMV infection remains a major complication after stem cell and solid organ transplantation. HCMV infected cells process HCMV proteins intracellularly and present HCMV-peptides on the cell surface in the context of HLA class I complexes. Antibodies naturally do not bind to peptides that are presented in HLA complexes and immunity against HCMV is mostly dependent on T-cells recognizing HLA I/HCMV-peptide complexes via their T-cell receptor. Antibodies that are generated to bind to peptides that are presented in HLA complexes are termed T-cell receptor like (TCR-like) antibodies. It has recently been shown that the T-cell response against an HLA-C*07:02 presented HCMV-peptide (CRVLCCYVL) derived from the IE-1 antigen is particularly strong. Also, HLA-C*07:02 is less susceptible to viral immune evasion than HLA-B and HLA-A molecules making it an attractive target for TCR-like antibodies. Here, we set out to obtain and characterize HLA-C*07:02 restricted, HCMV-specific antibodies.

Antibody Fab fragments (Fabs) specific for the HLA-C*07:02 restricted HCMV-peptide CRVLCCYVL were identified using phage display technology. For selection, phages were incubated for 1 hour with biotinylated HLA-C*07:02/CRVLCCYVL complexes at decreasing concentrations (15, 5 and 1 µg/ml). Before each round a pre-absorption using HLA-C*07:02 complexes folded with control-peptide was performed. Selected phages were eluted and used to infect E. coli bacteria of the strain TG1 to produce soluble Fabs, which were then tested for binding to HCMV-peptide (CRVLCCYVL) and control-peptide pulsed lymphoblastoid cell lines (LCLs) and lymphocytes expressing HLA-C*07:02. As control, selected Fabs were tested for binding to HCMV-peptide (CRVLCCYVL) and control-peptide loaded LCLs and lymphocytes expressing different HLA-C alleles.

By this, we were able to identify 3 different Fabs (4/4, 7/3 and 13/2) that bound to HLA-C*07:02 expressing LCLs and blood lymphocytes loaded with the HCMV-peptide CRVLCCYVL. All Fabs proofed to be highly specific since they showed no binding properties towards HLA-C*07:02 positive LCLs and lymphocytes that were loaded with no peptide or a control peptide. Furthermore, only LCLs and lymphocytes expressing the HLA-C*07:02 allele could be stained by our Fabs after HCMV-peptide loading. LCLs and lymphocytes of different HLA-C alleles that were pulsed with the HCMV-peptide CRVLCCYVL or a control peptide were not bound by the selected Fabs.

Using phage display technology, we identified 3 TCR-like antibody fragments that target specifically HLA-C*07:02 positive LCLs and lymphocytes when loaded with the IE-1 derived HCMV-peptide CRVLCCYVL. The Fabs 4/4, 7/3 and 13/2 seem to be highly specific and to our knowledge the described Fabs are the first TCR-like antibodies that target an HLA-C presented peptide. Further studies are underway to determine binding properties to HCMV infected fibroblasts and affinity of the identified HLA-C*07:02 restricted and HCMV-specific Fabs and to explore their therapeutic potential.