Th17 cells represent a distinct subset of T cells characterized by unique overall phenotype and function. Th17 cells are  now known to be derived from naïve CD4+ Th0 cells.  As with other T cell subsets Th17 cells mature under specific, tightly controlled conditions. An overall T cell polarization pathway is shown in Figure 1.  By phenotype Th17 cells are usually characterized by expression of cell surface CD4 and   IL-23 Receptor surface markers along with the prototypical intracellular Th17 transcription marker RORγ/RORγ(t).   Th17 cells also express IL-17 by intracellular staining. 

Figure 1. T Cell Subset Polarization Pathways and Response Initiated through Dendritic Cell Activation

Functionally Th17 cells produce and secrete IL-17 isoforms and pro-inflammatory cytokines in response to both exogenous and self antigens. IL-17 stimulates induction of other pro-inflammatory cytokines TNF alpha, IL-1 beta, IL-6, and IL-8 . Th17 cells play major roles in host defense an example schematic is shown in shown in Figure 1.  Th17 cells are also linked with pathogenic IL-17 inflammatory responses as effectors in the autoimmune process.in several chronic inflammatory diseases such as rheumatoid arthritis, psoriasis and multiple sclerosis.

Figure 2. Th17 Cell Polarization Pathways and Response Initiated through Dendritic Cell Activation

References:

Chen Z and O’Shea JJ 2008 Immunol Res 41: 87-102
Mangan PR et al 2006 Nature 441: 231-234
Veldhoen M et al 2006 Immunity 24:179-189

Rutitzky & Staedecker 2006 Mem Inst Oswaldo Cruz 101 Suppl 1: 326-330