posted on 2024-01-17, 15:39authored byKelli
A. McCord, Chao Wang, Mirjam Anhalt, Wayne W. Poon, Amanda L. Gavin, Peng Wu, Matthew S. Macauley
Fcγ receptors (FcγRs) play key roles in the
effector
function of IgG, but their inappropriate activation plays a role in
several disease etiologies. Therefore, it is critical to better understand
how FcγRs are regulated. Numerous studies suggest that sialic
acid-binding immunoglobulin-type lectins (Siglecs), a family of immunomodulatory
receptors, modulate FcγR activity; however, it is unclear of
the circumstances in which Siglecs can antagonize FcγRs and
which Siglecs have this ability. Using liposomes displaying selective
ligands to coengage FcγRs with a specific Siglec, we explore
the ability of Siglec-3, Siglec-5, Siglec-7, and Siglec-9 to antagonize
signaling downstream of FcγRs. We demonstrate that Siglec-3
and Siglec-9 can fully inhibit FcγR activation in U937 cells
when coengaged with FcγRs. Cells expressing Siglec mutants reveal
differential roles for the immunomodulatory tyrosine-based inhibitory
motif (ITIM) and immunomodulatory tyrosine-based switch motif (ITSM)
in this inhibition. Imaging flow cytometry enabled visualization of
SHP-1 recruitment to Siglec-3 in an ITIM-dependent manner, while SHP-2
recruitment is more ITSM-dependent. Conversely, both cytosolic motifs
of Siglec-9 contribute to SHP-1/2 recruitment. Siglec-7 poorly antagonizes
FcγR activation for two reasons: masking by cis ligands and
differences in its ITIM and ITSM. A chimera of the Siglec-3 extracellular
domains and Siglec-5 cytosolic tail strongly inhibits FcγR when
coengaged, providing evidence that Siglec-5 is more like Siglec-3
and Siglec-9 in its ability to antagonize FcγRs. Additionally,
Siglec-3 and Siglec-9 inhibited FcγRs when coengaged by cells
displaying ligands for both the Siglec and FcγRs. These results
suggest a role for Siglecs in mediating FcγR inhibition in the
context of an immunological synapse, which has important relevance
to the effectiveness of immunotherapies.