Top Reads

Engagement of TCR with its cognate Ag results in a conformational change that exposes the CD3 cytoplasmic domains and allows for phosphorylation via lymphocyte-specific protein tyrosine kinase (Lck). The phosphotyrosines pTyr³⁹⁴ and pTyr⁵⁰⁵ on Lck function as activating or inhibitory regulators, respectively. In this Top Read, Borowicz et al. (p. 1128) characterized pTyr¹⁹² in the Lck SH2 domain as an additional regulator by generating CRISPR/Cas9-targeted knock-in mutants expressing Lck Glu¹⁹² or Phe¹⁹² in Jurkat T cells. Lck Phe¹⁹² remained unphosphorylated and showed hyperactivation, but Lck Glu¹⁹² was hyperphosphorylated at Lck Tyr⁵⁰⁵, and a combined mutant targeting Lck Glu¹⁹² and Phe⁵⁰⁵ had elevated levels of pTyr³⁹⁴. These results suggest that Lck Tyr¹⁹² curbs Lck hyperactivation and limits its activity in resting cells.

Klebsiella pneumoniae is a Gram-negative bacteria associated with antibiotic resistant pneumonia, for which improved treatments are needed to manage emerging strains associated with more severe disease. In this Top Read, Rangasamy et al. (p. 1112) characterized the role of murine stem cell Ag-1⁺ lung mesenchymal stem cells (Sca-1⁺ LMSCs) in K. pneumoniae. They purified Sca-1⁺ LMSCs from murine lungs and observed that inflammatory cytokine production by macrophages and neutrophils can be modulated by Sca-1⁺ LMSCs upon LPS treatment in vitro. Sca-1⁺ LMSCs and neutrophils could inhibit K. pneumoniae growth in vitro, and each of these cell types demonstrated phagocytic and intracellular killing functions. Lung inflammation was reduced in K. pneumoniae infected mice upon treatment with Sca-1⁺ LMSCs and was associated with greater bacterial clearance and improved survival. These findings indicate that Sca-1⁺ LMSCs are key players in curbing the severity of K. pneumoniae–associated pneumonia.

The process of selection ensures that thymocytes expressing strongly autoreactive TCRs will either be killed by apoptosis or be redirected down an alternative developmental pathway. In this Top Read, Boulet et al. (p. 1055) defined a function of the orphan nuclear receptor NR4A3 during negative selection. Double-positive thymocytes with strong self-reactive signals showed upregulation of Nr4a3, and NR4A3-deficient mice accumulated CD44^hi single-positive CD8 thymocytes that had failed negative selection and could induce autoimmune responses. NR4A3-deficient mice also showed increased survival of single-positive CD4 T cells that should have undergone negative selection and had elevated production of regulatory T cells. These observations affirm previous studies that have suggested a role for the NR4A family in negative selection and limiting autoimmunity.

Edwardsiella piscicida is an intracellular bacterial pathogen that has had a major impact on farmed fish. However, little is known about the fish immune responses to E. piscicida. In this Top Read, Yin et al. (p. 1087) observed that E. piscicida accumulated in LC3⁺ autophagosomes and lysosomes in monocytes/macrophages from grass carp (Ctenopharyngodon idella). The nucleotide-binding oligomerization domain (NOD) 1 served as an intracellular sensor of E. piscicida and recruited ATG16L1 to the bacterial surface, which resulted in autophagy and bacterial clearance. Two E. piscicida virulence factors, EVPC and ESCB, were able to reduce NOD1 expression, but IFN-γ was associated with greater colocalization of NOD1 and E. piscicida, as well as increased autophagy-mediated bacterial clearance. These data define a role for NOD-1 mediated autophagy of E. piscicida in a defense mechanism used by fish for protection against intracellular pathogens.