Mannose-Capped Lipoarabinomannan from Mycobacterium tuberculosis Induces CD4+ T Cell Anergy via GRAIL

Abstract

Mycobacterium tuberculosis cell wall glycolipid, lipoarabinomannan, can inhibit CD4+ T cell activation by downregulating the phosphorylation of key proximal TCR signaling molecules: Lck, CD3ζ, ZAP70, and LAT. Inhibition of proximal TCR signaling can result in T cell anergy, in which T cells are inactivated following an Ag encounter, yet remain viable and hyporesponsive. We tested whether mannose-capped lipoarabinomannan (LAM)-induced inhibition of CD4+ T cell activation resulted in CD4+ T cell anergy. The presence of LAM during primary stimulation of P25 TCR-transgenic murine CD4+ T cells with M. tuberculosis Ag85B peptide resulted in decreased proliferation and IL-2 production. P25 TCR-transgenic CD4+ T cells primed in the presence of LAM also exhibited decreased response upon restimulation with Ag85B. The T cell anergic state persisted after the removal of LAM. Hyporesponsiveness to restimulation was not due to apoptosis, generation of Foxp3-positive regulatory T cells, or inhibitory cytokines. Acquisition of the anergic phenotype correlated with upregulation of gene related to anergy in lymphocytes (GRAIL) protein in CD4+ T cells. Inhibition of human CD4+ T cell activation by LAM also was associated with increased GRAIL expression. Small interfering RNA–mediated knockdown of GRAIL before LAM treatment abrogated LAM-induced hyporesponsiveness. In addition, exogenous IL-2 reversed defective proliferation by downregulating GRAIL expression. These results demonstrate that LAM upregulates GRAIL to induce anergy in Ag-reactive CD4+ T cells. Induction of CD4+ T cell anergy by LAM may represent one mechanism by which M. tuberculosis evades T cell recognition.