Modulation of Cord Blood CD8⁺ T Cell Effector Differentiation by 4-1BB and CD30: Cross-Talk with TGF-β1.

Transforming growth factor-β1 (TGF-β1), an immunosuppressive cytokine, potently suppressed cytotoxic T lymphocyte (CTL) differentiation of human cord blood naïve CD8⁺ T cells as determined by reduced induction of characteristic phenotypes of effector cells and cytotoxic activity. In contrast, 4-1BB (CD137), a costimulatory molecule in the TNF receptor family, promoted effector and memory CTL differentiation. We present evidence that the suppressive TGF-β1 effects can be effectively reversed by anti-4-1BB costimulnation during in vitro CTL differentiation driven by IL-15 from exclusively naïve cord blood CD8⁺ T cells. 4-1BB may counteract TGF-β1 effects at the level of TGF-β1 signaling by inhibiting Smad2 phosphorylation induced by TGF-β1. In contrast to 4-1BB, recent studies show that CD30, another member of the TNF receptor family, may play an important role in down-modulating effector or memory CTL responses. Thus, we hypothesized that CD30 may interact with 4-1BB in responding to the TGF-β1 suppression. TGF-β1 inhibits 4-1BB expression on CTLs. Conversely, TGF-β1 greatly up-regulates CD30 expression. IL-4 augments the TGF-β1-mediated up-regulation of CD30 expression resulting in further increase of CD30 expression. In contrast, IL-12 abrogates TGF-β1-induced CD30 expression. Such IL-4 and IL-12 effects on CD30 expression appear to be exactly opposite for 4-1BB expression: down- and up-regulating 4-1BB expression, respectively. Co-engagement of CD30 abrogates 4-1BB expression as well as its activity for reversing TGF-β1-mediated suppression of CTL proliferation particularly in the presence of IL-4. Expression of NKG2D, an important stimulatory receptor for NK and CTL cytotoxic activity is inhibited by TGF-β1. 4-1BB stimulation restores the NKG2D expression inhibited by TGF-β1. Again, co-engagement of CD30 significantly inhibits the 4-1BB activity for restoration of NKG2D expression. Taken together, our data suggest that 4-1BB promotes effector CTL differentiation and CD30 acts as a negative feed-back molecule, promoting TGF-β1-mediated CTL suppression and counteracting 4-1BB activity. In conclusion, we have delineated a novel link between CD30 expression and TGF-β1 which is pivotal to the down-regulation of 4-1BB-induced CTL effector differentiation.