Human CD4- CD8- Invariant Natural Killer T Cells Promote IgG Secretion from B Cells Stimulated by Cross-Linking of Their Antigen Receptors

Immunoglobulin (Ig) M production can be induced by the interaction of thymus-independent type-2 (TI-2) antigen (Ag) with B cell Ag receptors (BCRs) without the involvement of conventional T cells;for IgG production through the same process, however, a second signal is required. Previous studies have reported that invariant natural killer T (iNKT) cells may be responsible for the second signal involved in IgG production. In the present study, we addressed whether human iNKT cells could participate in the production of Ig against TI-2 Ag in vitro. Two major distinct subsets of human iNKT cells, CD4+ CD8β- (CD4) and CD4- CD8β- [double negative (DN)] cells, were generated from peripheral blood monocytes from a healthy volunteer. BCR engagement, triggered by anti-IgM antibody stimulation, examined here as a model of BCR engagement triggered by TI-2 Ag, induced abundant IgM production by B cells. Both CD4 and DN iNKT cells reduced IgM production and conversely enhanced IgG production in a dose-dependent manner. In addition, IgG production by CD19+CD27- (naïve) and CD19+CD27+ (memory) B cells was predominantly promoted by DNiNKT cells rather than CD4 iNKT cells;nevertheless, IgM production by both B cell subsets was similarly reduced by either subset of iNKT cells. These results suggest that the DN iNKT subsets may preferentially promote Ig class switching by B cells upon stimulation with TI-2 Ag.

SRSF1 plays a critical role in invariant natural killer T cell development and function

Invariant natural killer T(iNKT)cells are highly conserved innate-like T lymphocytes that originate from CD4^(+)CD8^(+)double-positive(DP)thymocytes.Here,we report that serine/arginine splicing factor 1(SRSF1)intrinsically regulates iNKT cell development by directly targeting Myb and balancing the abundance of short and long isoforms.Conditional ablation of SRSF1 in DP cells led to a substantially diminished iNKT cell pool due to defects in proliferation,survival,and TCRαrearrangement.The transition from stage 0 to stage 1 of iNKT cells was substantially blocked,and the iNKT2 subset was notably diminished in SRSF1-deficient mice.SRSF1 deficiency resulted in aberrant expression of a series of regulators that are tightly correlated with iNKT cell development and iNKT2 differentiation,including Myb,PLZF,Gata3,ICOS,and CD5.In particular,we found that SRSF1 directly binds and regulates pre-mRNA alternative splicing of Myb and that the expression of the short isoform of Myb is substantially reduced in SRSF1-deficient DP and iNKT cells.Strikingly,ectopic expression of the Myb short isoform partially rectified the defects caused by ablation of SRSF1.Furthermore,we confirmed that the SRSF1-deficient mice exhibited resistance to acute liver injury uponα-GalCer and Con A induction.Our findings thus uncovered a previously unknown role of SRSF1 as an essential post-transcriptional regulator in iNKT cell development and functional differentiation,providing new clinical insights into iNKT-correlated disease.

MicroRNAs are key regulators controlling iNKT and regulatory T-cell development and function

MicroRNAs(miRNAs)are an abundant class of evolutionarily conserved,small,non-coding RNAs that post-transcriptionally regulate expression of their target genes.Emerging evidence indicates that miRNAs are important regulators that control the development,differentiation and function of different immune cells.Both CD4^(+)CD25^(+)Foxp3^(+) regulatory T(Treg)cells and invariant natural killer T(iNKT)cells are critical for immune homeostasis and play a pivotal role in the maintenance of self-tolerance and immunity.Here,we review the important roles of miRNAs in the development and function of iNKT and Treg cells.