TREM2 and infection: Interestingly, infection with Listeria monocytogenes, a bacterium that translocates to the cytosol, only mildly increased TREM2 expression at 5 h postinfection, suggesting that mycobacterium-specific products and/or host factors activated at later phases of infection may be required to trigger the upregulation of TREM2 (Fig. 2F).