4-1BB IS EXPRESSED ON CD45RA(HI)RO(HI) TRANSITIONAL T-CELL IN HUMANS

Murine 4-1BB is an similar to 30-kDa glycoprotein expressed on activat ed T cells and plays a role in T-cell-mediated proliferative response. To date the majority of work on 4-1BB has been conducted in the mouse . To assess the role of 4-1BB in humans, mAbs were made against the re combinant human (rh) 4-1BB protein. One such mAb 4B4-1 specifically bi nds SF-21 insect cells expressing rh4-1BB but not irrelevant control p rotein as measured by how cytometry (FCM). 4B4-1 mAb stains PMA- and i onomycin-stimulated CEM (human T lymphoma) cells and PHA-stimulated pe ripheral blood T cells, but not resting cells. 4B4-1 mAb immunoprecipi tates both similar to 32 and similar to 80 kDa protein from rh4-1BB ex pressing SF-21 cells and an similar to 39- and similar to 85-kDa prote in from PMA-stimulated CEM cells under reducing conditions by SDS-PAGE . As added proof of its specificity, binding of FITC-labeled 4B4-1 mAb to PHA-stimulated T cells was blocked by rh4-1BB protein. Together th ese data demonstrate that 4B4-1 is specific for 4-1BB in humans. Unlik e in the mouse, 4-1BB is expressed much earlier (within 24 hr) peaking around 2-3 days following PHA stimulation. As in the mouse 4-1BB is i nduced on both CD4(+) and CD8(+) T cell subsets. 4-1BB expression is i nduced upon PHA stimulation in both the naive (CD45RA(hi)-CD45RO(lo/-) ) and the memory (CD45RA(lo/-)-RO(hi)) cell populations. Virtually all CD45RA(hi)RO(lo/-) cells upon culture in PHA give rise to an intermed iate CD45RA(hi)RO(hi) 4-1BB(+) transitional cell and subsequently CD45 RA(lo/-)RO(hi) 4-1BB(lo/-) and CD45RA(lo/-)RO(hi) 4-1BB(hi) cells. In contrast, approximately 27% of CD45RA(lo/-)RO(hi) 4-1BB(-) cells when cultured in PHA for 24 hr acquire 4-1BB expression and all remain CD45 RA(lo/-)RO(hi). (C) 1996 Academic Press, Inc.