PBX1-d is novel splice isoform of pre-B-cell leukemia homeobox 1 (PBX1) that lacks
its DNA-binding and Hox-binding domains, and functions as a dominant negative. We
have shown that PBX1-d expression in CD4(+) T cells is associated with systemic lupus
erythematosus (SLE) in a mouse model as well as in human subjects. More specifically,
PBX1-d expression leads to the production of autoreactive activated CD4+ T cells,
a reduced frequency and function of Foxp3+ regulatory T (Treg) cells and an expansion
of follicular helper T (Tfh) cells. Very little is known about the function of PBX1
in T cells, except that it directly regulates the expression of miRNAs associated
with Treg and Tfh homeostasis. In the present study, we show that PBX1 directly regulated
the expression of CD44, a marker of T cell activation. Two PBX1 binding sites in the
promoter directly regulated CD44 expression, with PBX1-d driving a higher expression
than the normal isoform PBX1-b. In addition, mutations in each of the two binding
sites had different effects of PBX1-b and PBX1-d. Finally, we showed that an enhanced
recruitment of co-factor MEIS by PBX1-d over PBX1-b, while there was no difference
for co-factor PREP1 recruitment. Therefore, this study demonstrates that the lupus-associated
PBX1-d isoform directly transactivates CD44, a marker of CD44 activation and memory,
and that it has different DNA binding and co-factor recruitment relative to the normal
isoform. Taken together, these results confirm that PBX1 directly regulates genes
related to T cell activation and shows that the lupus-associated isoform PBX1-d has
unique molecular functions.