Riboflavin Metabolism Variation among Clinical Isolates of Streptococcus pneumoniae Results in Differential Activation of Mucosal-associated Invariant T Cells

Abstract

Streptococcus pneumoniae is an important bacterial pathogen that causes a range of noninvasive and invasive diseases. The mechanisms underlying variability in the ability of S. pneumoniae to transition from nasopharyngeal colonization to disease-causing pathogen are not well defined. Mucosal-associated invariant T (MAIT) cells are prevalent in mucosal tissues such as the airways and are believed to play an important role in the early response to infection with bacterial pathogens. The ability of MAIT cells to recognize and contain infection with S. pneumoniae is not known. In the present study, we analyzed MAIT-cell responses to infection with clinical isolates of S. pneumoniae serotype 19A, a serotype linked to invasive pneumococcal disease. We found that although MAIT cells were capable of responding to human dendritic and airway epithelial cells infected with S. pneumoniae, the magnitude of response to different serotype 19A isolates was determined by genetic differences in the expression of the riboflavin biosynthesis pathway. MAIT-cell release of cytokines correlated with differences in the ability of MAIT cells to respond to and control S. pneumoniae in vitro and in vivo in a mouse challenge model. Together, these results demonstrate first that there are genetic differences in riboflavin metabolism among clinical isolates of the same serotype and second that these likely determine MAIT-cell function in response to infection with S. pneumoniae. These differences are critical when considering the role that MAIT cells play in early responses to pneumococcal infection and determining whether invasive disease will develop.

An overview on the identification of MAIT cell antigens

“…Typhimurium inhibited the activation of Jurkat.MAIT cells by these culture supernatants . Consistent with our findings, exogenous riboflavin has also recently been shown to decrease activation of MAIT cells by Streptococcus pneumoniae . In the S .…”

“…In these, and other gram-positive bacteria, including Lactococcus spp., the Rib genes are grouped into a single, 4-gene operon (ribGBAH), which is under transcriptional regulation through a riboswitch, such that riboflavin or flavin mononucleotides (FMN) downregulate the further production of riboflavin. 43,44 Two recent papers demonstrate MAIT cell activation by Streptococcus pneumoniae and show that the riboflavin biosynthesis pathway is highly conserved among pneumococci and also present in other Streptococcus spp., 45,46 but lacking from 2400 S. pyogenes strains, 46 consistent with a previously reported lack of MAIT cell activation. 21 In 571 pneumococci the genes ("rib DEAH", equivalent to "rib GBAH") were found clustered together in a highly conserved operon.…”

“…Two recent papers demonstrate MAIT cell activation by Streptococcus pneumoniae and show that the riboflavin biosynthesis pathway is highly conserved among pneumococci and also present in other Streptococcus spp., but lacking from 2400 S. pyogenes strains, consistent with a previously reported lack of MAIT cell activation . In 571 pneumococci the genes (“ rib DEAH”, equivalent to “ rib GBAH”) were found clustered together in a highly conserved operon …”

“…42 Consistent with our findings, exogenous riboflavin has also recently been shown to decrease activation of MAIT cells by Streptococcus pneumoniae. 45,46 In the S. Typhimurium genome, ribD and ribH are grouped togetherthus initial experiments demonstrating the requirement for the riboflavin pathway in the production of MAIT cell stimulatory Ags used a mutant lacking both genes. S. Typhimurium SL1344 DribDH, 42 and later S. Typhimurium BRD509 DribDH, 19 demonstrated markedly decreased MAIT cell activation, and this activity could be reconstituted by addition to the mutants of ribDH expressed from a plasmid.…”

MAIT cells and microbial immunity

“…In these and other subsequent studies, it was demonstrated that for some microbes including S. enterica typhimurium 9,15 , L. lactis 9 , E. coli 16 , and Streptococcus pneumoniae ( S. pneumoniae ) 17 , an ability to synthesize riboflavin through functional Rib enzymes was critical to MR1-dependent recognition of these microbes (Table 1). Specifically, for these bacterial species, the RibD enzyme was required for production of 5-amino-6-D-ribitylaminouracil (5-A-RU), a precursor to riboflavin, and a critical component of these ligands.…”