Cereblon is a novel E3 ubiquitin ligase regulator of CD28 signaling in T cells (P1007)

Abstract E3 ubiquitin ligases (E3-UbL) regulate clonal anergy and immune self-tolerance. Of the known regulators, c-Cbl and Cbl-b regulate T-cell growth factor production and proliferation induced by IL-2 through the CD28 pathway. Investigations of immunomodulatory (IMiDsTM) drugs lenalidomide (LEN) and thalidomide in myelodysplastic syndrome and multiple myeloma show that these analogs are capable of activating T-cells in the absence of co-stimulation and reverse T-cell anergy. LEN increased IL-2 production (p<0.001), proliferation (p<0.001), and pCREB promoter binding in primary human T-cells after TCR stimulation alone without CD28-receptor ligation. Comparing LEN-induced response in sorted human CD28+CD8+ and CD28-CD8+ T-cells and in CD28 shRNA knock-down T-cells indicates that LEN acts to directly potentiate receptor function dependent on CD28 expression. A landmark study identified cereblon (CRBN), a RING-domain E3-UbL as the primary target of thalidomide. CRBN has also been shown to mediate LEN immunomodulatory activity in vitro. To examine the effect of CRBN on CD28 signaling, germ-line CRBN knock-out mice (crbn-/-) were studied and found to have normal numbers of mature T-cells, normal naïve and memory compartmentalization, and normal proliferative response, but increased contextual activation of IL-2 in the absence of CD28 ligation. These results reveal a novel E3-UbL regulator involved in CD28 signaling with importance in anti-tumor immunity.