The T/NK cell co-stimulatory molecule SECTM1 is an IFN "early response gene" that is negatively regulated by LPS in human monocytic cells

Background: SECTM1 is a T/NK cell "co-stimulatory" molecule that is expressed in the peripheral blood by neutrophils and monocytes.

Methods: We used qRT-PCR to investigate the mRNA expression of SECTM1 in human monocytic cells after stimulation with interferons and LPS and confirmed the protein expression by flow cytometry.

Results: The kinetics of interferon induced SECTM1 mRNA expression in MM6 cells are time dependent occurring rapidly within 3h of stimulation and reaching a maximal level at ~6h for IFN-α and ~12h for IFN-β and IFN-γ. Co-treatment of MM6 cells with IFN-γ and cycloheximide caused a superinduction of SECTM1 mRNA expression while cycloheximide alone had no effect, illustrating that de novo protein synthesis is not required for IFN-γ enhanced expression of SECTM1 mRNA, a characteristic of IFN early response genes. The kinetics of IFN induced SECTM1 mRNA expression in primary monocytes is comparable although it occurs much quicker with rapid induction by IFN-α, IFN-β and IFN-γ and maximal levels reached in <6h. Human monocytic cells also displayed a pronounced negative regulation of SECTM1 mRNA expression by LPS, while at the protein level SECTM1 expression was also shown to be regulated by IFN and LPS. Bioinformatic analysis of the SECTM1 promoter region identified STAT1α/GAS, STAT3, ISRE, NFκB and putative p63 binding sites suggesting a complex transcriptional control. This tight regulation of SECTM1 gene expression and rapid upregulation highlights its relevance in the innate immune response.

Conclusion: Human monocytes produce SECTM1 in response to interferon stimuli that is negatively regulated by LPS.

General significance: The level of SECTM1 expression is likely to be a key factor in innate immune responses and in the immune tolerance of cancerous cells.