# Ligand requirements for immunoreceptor triggering

## TL;DR

The study shows that immunoreceptor activation depends on ligand size, not mobility or valency, supporting the kinetic-segregation model.

## Contribution

The paper introduces a generic ligand system to demonstrate that size-based segregation drives immunoreceptor triggering.

## Key findings

- Increasing ligand length impairs activation despite enhancing cell-cell conjugation.
- Ligand valency enhances conjugation but not activation at equivalent conjugation levels.
- Findings support the kinetic-segregation model over clustering or mechanical force.

## Abstract

Leukocytes interact with other cells using cell surface receptors. The largest group of such receptors are non-catalytic tyrosine phosphorylated receptors (NTRs), also called immunoreceptors. NTR signalling requires phosphorylation of cytoplasmic tyrosine residues by SRC-family tyrosine kinases. How ligand binding to NTRs induces this phosphorylation, also called NTR triggering, remains controversial, with roles suggested for size-based segregation, clustering, and mechanical force. Here we exploit a recently developed cell-surface generic ligand system to explore the ligand requirements for NTR triggering. We examine the effect of varying the ligand’s length, mobility and valency on the activation of representative members of four NTR families: SIRPβ1, Siglec 14, NKp44 and TREM-1. Increasing the ligand length impairs activation via NTRs, despite enhancing cell-cell conjugation, while varying ligand mobility has little effect on either conjugation or activation. Increasing the valency of the ligand, while enhancing cell-cell conjugation, does not enhance activation at equivalent levels of conjugation. These findings are more consistent with a role for size-based segregation, rather than mechanical force or clustering, in NTR triggering, suggesting a role for the kinetic-segregation model.

Using a generic ligand system, we show that activation via immunoreceptors is dependent on the size, but not the valency or mobility, of their cell surface ligand. This suggests that they trigger using the kinetic segregation mechanism.