Tissue Non-specific Alkaline Phosphatase Expression is Needed for the Full Stimulation of T Cells and T Cell-Dependent Colitis.

Background and Aims: Two alkaline phosphatase isoforms, intestinal [IAP] and tissue non-specific alkaline phosphatase [TNAP], are coexpressed in mouse colon, with the latter predominating in colitis. We aimed to examine the role of TNAP in T lymphocytes, using heterozygous TNAP+/- mice [as TNAP-/- mice are non-viable].

Methods: In vitro primary cultures and in vivo T cell models using TNAP+/- mice were used.

Results: Stimulated splenocytes [lipopolysaccharide and concanavalin A] and T lymphocytes [concanavalin A and a-CD3/a-CD28] showed a decreased cytokine production and expression when compared with wild-type [WT] cells. Decreased T cell activation was reproduced by the TNAP inhibitors levamisole, theophylline, and phenylalanine in WT cells. Intraperitoneal administration of anti-CD3 in vivo resulted in reduced plasma cytokine levels, and decreased activation of splenocytes and T cells ex vivo in TNAP+/- mice. We further tested the hypothesis that TNAP expressed in T lymphocytes is involved in T cell activation and inflammation, using the lymphocyte transfer model of colitis. Rag1-/- mice were transferred with T naïve cells [CD4+ CD62L+] from TNAP+/- or WT mice and developed colitis, which was attenuated in the group receiving TNAP+/- cells. Compared with WT, T cells from TNAP+/- mice showed a decreased capacity for proliferation, with no change in differentiation.

Conclusions: Our results offer clear evidence that TNAP modulates T lymphocyte function and specifically T cell-dependent colitis. This was associated with distinct changes in the type of TNAP expressed, probably because of changes in glycosylation.