<p>Supporting dataset.</p>

However, its receptor and the mechanisms underlying its immunosuppression remain poorly understood. Here, we identify Galectin-9 (Gal-9) as a binding partner of B7-H4 and investigate its role in modulating T cell responses.

We show that glycosylation within the IgC domain of B7-H4 is required for Gal-9 binding, while the N-terminal carbohydrate recognition domain (N-CRD) of Gal-9—specifically residue R65—is essential for its binding with B7-H4. In addition, several other B7 family members (B7.1, B7.2, B7-H2, and B7-DC) and immune cell surface receptors (CD28, 2B4, CD226, and SLAMF1) also bind to Gal-9 at levels comparable to those observed with B7-H4 or TIM-3. <i>In vitro</i> functional assays revealed that B7-H4 inhibits Gal-9-induced activation of CD28 downstream signaling and reduces Gal-9-mediated T cell death. <i>In vivo</i>, Gal-9 deficiency in mice resulted in an increased proportion of splenic CD4⁺ T cells, whereas B7-H4 deficiency produced no detectable phenotype.