Immune checkpoint therapy exemplified by blockade of the PD-1:PD-L1 pathway represents a promising cancer treatment. Nonetheless, a large number of patients and cancer types do not respond well to the therapy.
We generated human PD-L1 (huPD-L1) binding sdAbs and evaluated them as a tool to perform noninvasive immune imaging of huPD-L1pos tumors. In addition, we used them to block the interaction between huPD-1:huPD-L1 as such enhancing the de novo activation of antigen-specific T cells and augmenting their functionality when interacting with tumor cells.
HuPD-L1 binding sdAbs were generated through immunization of alpaca’s with human and mouse PD-L1. The resulting sdAbs were characterized, evaluating binding on recombinant huPD-L1 by ELISA and on huPD-L1-expressing cells by flow cytometry, and determining their affinity and blocking capacity using surface plasmon resonance. Selected high affinity sdAbs were labeled with Technetium-99m (99mTc) and used for SPECT/CT imaging of naïve mice or mice bearing either MCF7 breast cancer cells or 624-mel melanoma cells that were lentivirally engineered to express huPD-L1. Ex vivo analysis was performed to assess the 99mTc-anti-huPD-L1 sdAb uptake (γ-counting) and PD-L1 expression (flow cytometry). Furthermore, these tumor cell lines or PD-L1 positive dendritic cells (DCs) were co-cultured with human CD8+ T cells, either or not in combination with anti-PD-L1 mAbs or sdAbs, whereafter we evaluated the amount of antigen-specific T cells and their functionality.
Out of 42 sdAbs, one sequence family, represented by sdAbs K1, K2, K3 and K4, was shown to have high affinity for hu-PD-L1. K2 was selected as lead compound for further evaluation of huPD-L1 imaging in xenograft cancer models. SPECT/CT and dissection analysis in immunodeficient mice bearing huPD-L1-modified or nonmodified MCF7 or 624-mel cells showed high uptake in both huPD-L1pos tumors, which correlated with huPD-L1 expression levels determined by flow cytometry.
We furthermore showed that adding Nb K2 to immature antigen (gp100 or melA) presenting dendritic or tumor cells, co-cultured with T cells, restores T cell receptor (TCR) triggering and leads to a higher number of antigen specific T cells, better proliferation and higher IFN-g secretion.
These data show that sdAb K2 holds promise to noninvasively assess PD-L1 expression levels in tumors and that it is able to block the interaction between PD-1 and PD-L1 leading to enhanced T-cell activity.