In the development of PD-L1-blocking therapeutics, it is essential to transfer initial in vitro findings into proper in vivo animal models. Classical immunocompetent mice are attractive due to high accessibility and low experimental costs. However, it is unknown whether inter-species differences in PD-L1 sequence and structure would allow for human-mouse cross applications. Here, we disclose the first structure of the mouse (m) PD-L1 and analyze its similarity to the human (h) PD-L1. We show that mPD-L1 interacts with hPD-1 and provides a negative signal toward activated Jurkat T cells. We also show major differences in druggability between the hPD-L1 and mPD-L1 using therapeutic antibodies, a macrocyclic peptide, and small molecules. Our study indicates that while the amino acid sequence is well conserved between the hPD-L1 and mPD-L1 and overall structures are almost identical, crucial differences determine the interaction with anti-PD-L1 agents, that cannot be easily predicted in silico.

The protein p53, known as the “Guardian of the Genome”, plays an important role in maintaining DNA integrity, providing protection against cancer-promoting mutations. Dysfunction of p53 is observed in almost every cancer, with 50% of cases bearing loss-of-function mutations/deletions in the TP53 gene. In the remaining 50% of cases the overexpression of HDM2 (mouse double minute 2, human homolog) protein, which is a natural inhibitor of p53, is the most common way of keeping p53 inactive. Disruption of HDM2-p53 interaction with the use of HDM2 antagonists leads to the release of p53 and expression of its target genes, engaged in the induction of cell cycle arrest, DNA repair, senescence, and apoptosis. The induction of apoptosis, however, is restricted to only a handful of p53^wt cells, and, generally, cancer cells treated with HDM2 antagonists are not efficiently eliminated. For this reason, HDM2 antagonists were tested in combinations with multiple other therapeutics in a search for synergy that would enhance the cancer eradication. This manuscript aims at reviewing the recent progress in developing strategies of combined cancer treatment with the use of HDM2 antagonists.

The p53 protein is engaged in the repair of DNA mutations and elimination of heavily damaged cells, providing anticancer protection. Dysregulation of p53 activity is a crucial step in carcinogenesis. This dysregulation is often caused by the overexpression of negative regulators of p53, among which MDM2 is the most prominent one. Antagonizing MDM2 with small molecules restores the activity of p53 in p53 wild‐type (p53^wt) cells and thus provides positive outcomes in the treatment of p53^wt cancers. Previously, we have reported the discovery of a panel of fluoro‐substituted indole‐based antagonists of MDM2. Here, we demonstrate the biological activity and stereoselectivity of the most active compound from this series. Both enantiomers of the esterified form of the compound, as well as its corresponding carboxylic acids, were found active in fluorescence polarization (FP) assay, nuclear magnetic resonance (NMR) and microscale thermophoresis (MST) assay, with K_i and K_D values around 1 μm. From these four compounds, the esterified enantiomer (R)‐5a was active in cells, which was evidenced by the increase of p53 levels, the induced expression of p53‐target genes (CDKN1A and MDM2), the selective induction of cell cycle arrest, and selective growth inhibition of p53^wt U‐2 OS and SJSA‐1 compared to p53^del SAOS‐2 cells. The analysis of the crystal structure of human MDM2 in complex with the compound (R)‐6a (carboxylic acid of the active (R)‐5a compound) revealed the classical three‐finger binding mode. Altogether, our data demonstrate the activity of the compound and provide the structural basis for further structure optimization.

IgM is a multivalent antibody which evolved as a first line defense of adaptive immunity. It consists of heavy and light chains assembled into a complex oligomer. In mouse serum there are two forms of IgM, a full-length and a truncated one. The latter contains μ’ chain, which lacks a variable region. Although μ’ chain was discovered many years ago, its origin has not yet been elucidated. Our results indicate that μ’ chain is generated from a full-length heavy chain by non-enzymatic cleavage of the protein backbone. The cleavage occurred specifically after Asn209 and is prevented by mutating this residue into any other amino acid. The process requires the presence of other proteins, preferentially with an acidic isoelectric point, and is facilitated by neutral or alkaline pH. This unique characteristic of the investigated phenomenon distinguishes it from other, already described, Asn-dependent protein reactions. A single IgM molecule is able to bind up to 12 epitopes via its antigen binding fragments (Fabs). The cleavage at Asn209 generates truncated IgM molecules and free Fabs, resulting in a reduced IgM valence and probably affecting IgM functionalityin vivo.