Cristian Camilo Galindo

Inhibitory receptors like PD-1 and CTLA-4 contribute to T cell dysfunction in cancer^1,2,3. Monoclonal antibodies (mAbs) blocking the interactions in trans of these receptors with their ligands on cancer cells or in the tumour microenvironment lead to clinical responses in some but not all types of cancer. Signalling lymphocytic activation molecule 6 (SLAMF6, also known as Ly108) is a homotypic receptor preferentially expressed on progenitor or stem-like exhausted T (T_pex) cells, but not on terminally exhausted T (T_ex) cells, as demonstrated in mouse models^4,5,6,7,8,9. In contrast to T_ex cells, T_pex cells retain the capacity for functional restoration after immune checkpoint blockade^10,11,12. The role of SLAMF6 in T cells remains ambiguous, as it has both activating and inhibitory effects, complicating its evaluation as a therapeutic target. Here we find that SLAMF6 was triggered in cis by homotypic interactions at the T cell surface. These interactions elicited inhibitory effects that suppressed activation of T cells and limited anti-tumour immunity, independently of SLAMF6 expression on tumour cells. mAbs against human SLAMF6 with a robust ability to disrupt the cis interactions strongly augmented T cell activation, reduced the proportions of exhausted T cells and inhibited tumour growth in vivo. Collectively, these findings show that SLAMF6 functions exclusively as a T cell inhibitory receptor, which is triggered by cis homotypic interactions. They also position SLAMF6 as a promising target for therapies aimed at enhancing anti-tumour immunity, regardless of SLAMF6 expression on tumour cells.

The ability of macrophages to eliminate cancer cells through phagocytosis is tightly controlled by pro-phagocytic and inhibitory phagocytic receptors. Pro-phagocytic receptors such as Fc receptors, macrophage-1 antigen (MAC-1; also known as CD11b/CD18) and signalling lymphocytic activation molecule family member 7 (SLAMF7) have been shown to facilitate tumour cell elimination in pre-clinical models, and Fc receptors have been determined critical for the anti-tumour efficacy of several monoclonal antibodies in the clinic. Pre-clinical and early clinical studies have also highlighted that blocking of inhibitory phagocytic checkpoints, most prominently signal-regulatory protein α (SIRPα) and its ligand CD47, is a promising therapeutic approach for cancer. However, concerns about limited efficacy and toxicities in recent clinical trials have led to diminished enthusiasm for this approach. In this Review, we examine the evidence supporting phagocytic checkpoints as targets for cancer therapy, while highlighting current challenges associated with this therapeutic strategy. We also offer recommendations for enhancing the efficacy and safety of this approach in future work.

Signal regulatory protein α (SIRPα) is a macrophage inhibitory receptor that limits phagocytosis and antitumor activity by interacting in trans with CD47 on tumor cells. Here, we found that a component of SIRPα’s inhibitory function occurred independently of CD47. Inhibition occurred because of interactions between SIRPα and CD18 (β ₂ integrin) in cis on the surface of macrophages, involving SIRPα amino acids distinct from those implicated in the SIRPα-CD47 interaction. This cis interaction prevented activation of CD18, which is necessary for phagocytosis. The combined blockade of SIRPα-CD18 and SIRPα-CD47 was essential for maximizing phagocytosis and suppression of tumor growth in vivo. Thus, inhibitory immune checkpoints such as SIRPα suppress cell activation through a mechanism targeting CD18 in cis, which occurs in addition to engagement by their inhibitory checkpoint ligands in trans. This dual mode of action should be considered when developing inhibitory checkpoint blockades for immunotherapy.

Cancer cells often overexpress CD47, which triggers the inhibitory receptor SIRPα expressed on macrophages, to elude phagocytosis and antitumor immunity. Pharmacological blockade of CD47 or SIRPα is showing promise as anticancer therapy, although CD47 blockade has been associated with hematological toxicities that may reflect its broad expression pattern on normal cells. Here we found that, in addition to triggering SIRPα, CD47 suppressed phagocytosis by a SIRPα-independent mechanism. This mechanism prevented phagocytosis initiated by the pro-phagocytic ligand, SLAMF7, on tumor cells, due to a cis interaction between CD47 and SLAMF7. The CD47–SLAMF7 interaction was disrupted by CD47 blockade and by a first-in-class agonist SLAMF7 antibody, but not by SIRPα blockade, thereby promoting antitumor immunity. Hence, CD47 suppresses phagocytosis not only by engaging SIRPα, but also by masking cell-intrinsic pro-phagocytic ligands on tumor cells and knowledge of this mechanism may influence the decision between CD47 blockade or SIRPα blockade for therapeutic purposes.

Monocytes and macrophages are involved in a wide range of biological processes and parasitic diseases. The characterization of the molecular mechanisms governing such processes usually requires precise control of the expression of genes of interest. We implemented a tetracycline-controlled gene expression system in the U937 cell line, one of the most used in vitro models for the research of human monocytes and macrophages. Here we characterized U937-derived cell lines in terms of phenotypic (morphology and marker expression) and functional (capacity for phagocytosis and for Leishmania parasite hosting) changes induced by phorbol-12-myristate-13-acetate (PMA). Finally, we provide evidence of tetracycline-inducible and reversible Lamin-A gene silencing of the PMA-differentiated U937-derived cells.

Hematopoietic progenitor cell (HPC) transplantation is a treatment option for malignant and nonmalignant diseases. Umbilical cord blood (UCB) is an important HPC source, mainly for pediatric patients. It has been demonstrated that human leukocyte antigen (HLA) matching and cell dose are the most important features impacting clinical outcomes. However, UCB matching is performed using low resolution HLA typing and it has been demonstrated that the unnoticed mismatches negatively impact the transplant. Since we found differences in CD34⁺ viability after thawing of UCB units matched for two different patients (p = 0.05), we presumed a possible association between CD34⁺ cell viability and HLA. We performed a multivariate linear model (n = 67), comprising pre-cryopreservation variables and high resolution HLA genotypes separately. We found that pre-cryopreservation red blood cells (RBC), granulocytes, and viable CD34⁺ cell count significantly impacted CD34⁺ viability after thawing, along with HLA-B or -C (R² = 0.95, p = 0.01; R² = 0.56, p = 0.007, respectively). Although HLA-B*40:02 may have a negative impact on CD34⁺ cell viability, RBC depletion significantly improves it.