Abstract
Purpose: :
Thrombospondin-1 (TSP-1) is a critical molecule for ocular immune privilege. Contribution by ocular APC-derived TSP-1 may be indirect by facilitating activation of latent TGFβ via CD36 ligation on their cell surface. We now investigate if TSP-1 can directly induce regulatory immune response independently of its TGFβ-activating property.
Methods: :
Thioglycolate-elicited peritoneal exudates cells from WT or CD36-/- mice were used as APCs. These cells, pulsed with Ovalbumin, were cultured overnight in the presence or absence of TGFβ (5 ng/ml) before co-culturing with CD4+CD25- T cells from OT-II mice. Expression of FoxP3 was evaluated in CD4+CD25+ T cells by flow cytometry. Similarly expression of FoxP3 in CD4+CD25- T cells from C57BL/6 (WT) and TSP1 null mice was assessed after stimulating them with anti-CD3 in the presence of TSP-derived peptide (4N1K) or control peptide. Peptide 4N1K was derived from the CD47 binding C-terminal domain of TSP-1 that lacks latent TGFβ-binding amino acid sequence. In some experiments blocking anti-CD47 antibody was used to prevent CD47 ligation by the 4N1K.
Results: :
The expression of TSP on the surface of TGFβ-exposed CD36-/- APCs was significantly reduced as compared to similarly treated WT APCs. Upon co-culture with CD4+CD25- OT-II T cells these CD36-/- APCs induced significantly reduced numbers of Foxp3+ve cells as compared to the WT controls. The Foxp3 induction was not abolished completely suggesting partial contribution of the cell bound TSP to the induction of a regulatory response. Activation CD4+CD25- cells in the presence of TSP-1 derived peptide devoid of TGFβ-activating sequence (4N1K) significantly increased Foxp3+ve cells as compare to the control peptide (4NGG). Similar increased Foxp3 expression was detectable in TSP-1 deficient T cells. However, 4N1K driven increase in Foxp3 expression was abrogated in the presence of anti-CD47 antibody against a TSP-1 receptor on T cells.
Conclusions: :
Cell surface CD36-bound TSP together with its soluble form produced by TGFβ-exposed ocular APCs promotes induction of a regulatory immune response associated with ocular immune privilege. The soluble TSP mediates its effect via ligation of CD47 on T cells independent of TGFβ activation. Thus TSP-1 contributes to ocular immune privilege independently of its ability to activate latent TGFβ.
Keywords: ACAID • immunomodulation/immunoregulation • flow cytometry