FOXP3 is a master regulator of immune homeostasis expressed specifically in CD4⁺ CD25⁺ T regulatory cells controlling their growth, development and function.  FOXP3 significance in the normal development of Tregs is better elucidated with the fact that mutated FOXP3 results in a rare and fatal early onset autoimmune disorder in humans called XLAAD/IPEX (human immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome), a condition phenotypically similar to Scurfy in mice. 

 

FOXP3 is primarily an oligomeric, transcriptional repressor protein that belongs to the P subfamily of forkhead (FKH)-winged helix family of transcriptional factors. Members of this subfamily are forkhead (FKH) box proteins characterized by the presence of a highly conserved C-terminal winged-helix/FKH DNA binding domain (DBD) and centrally-located C2H2 zinc finger domain and leucine zipper domain.  Apart from these, an additional N-terminal proline rich region is present in FOXP3, whose function is yet to be understood. Studies have shown that FOXP3 is a nuclear-localized protein that specifically trans-represses NF-AT-induced expression of cytokines and other transcriptional factors in Tregs including IL-2, IL-4, IFN-gamma and NF-κB.

 

However, FOXP3 is not the sole master switch regulating the origin and development of CD4⁺ CD25⁺ Tregs. Studies have confirmed the existence of splice variant forms FOXP3 that are specifically expressed in humans but are lacking in mouse. Cloning and RT-PCR analysis from mRNA of CD4⁺ CD25⁺ T regulatory cells (Allan et. al, Smith et. al) has shown that these cells express two different alternatively spliced variant forms of FOXP3. While the FOXP3Δ2 variant had a deleted 105bp exon2 region, there was another FOXP3Δ2, Δ7 variant that had an additional 81bp exon7 deletion apart from exon2 deletion.

The predicted molecular weight of this FOXP3Δ2 is ~4KDa lower than the molecular weight of FOXP3.  Transient transfection assays using Jurkat cells suggest that the FOXP3Δ2 is novel splice variant that functions as a transcriptional repressor protein and acts in cohort with FOXP3 causing a significant suppression of cytokines and up-regulating the expression of various Treg-associated markers.

 

The existence of the splice variant forms of FOXP3 protein suggests an additional level of complexity related to the biology of FOXP3.  A lot research needs to be done so as to elucidate the physiological and functional importance of FOXP3 splice variant forms towards maintaining immune homeostasis in Tregs and preventing autoimmune disorders.

 

Continuing its efforts towards supporting research on FOXP3, IMGENEX has come up with an antibody that specifically recognizes the FOXP3Δ2 splice variant.