AN ANTIBODY TO CD44 ENHANCES HEMATOPOIESIS IN LONG-TERM MARROW CULTURES

Monoclonal antibody S5, which is specific for CD44, facilitates engraf tment of major histocompatibility complex (MHC)-mismatched canine bone marrow (BM) when infused into recipient animals before total body irr adiation (TBI) and marrow infusion. The precise mechanism by which S5 facilitates engraftment is not known. Previously published data in a m urine long-term bone marrow culture (LTBMC) model with other anti-CD44 monoclonal antibodies (mAbs) demonstrated an abrogation of myelo- and lymphopoiesis in LTBMC. To address this issue in an in vitro setting, the effect of S5 on canine myelopoiesis in LTBMC was investigated. Th e data indicate that treatment of LTBMC with S5 causes an absolute inc rease in the number of progenitor cells as measured by a colony-formin g unit-granulocyte/macrophage (CFU-GM) assay compared to cultures trea ted with control mAb (p < 0.0001). This effect was not observed with t hree other anti-CD44 mAbs used (Hermes-1, S3, and IM7). In addition, a concomitant decrease in the production of nonadherent cells was noted (p < 0.0001). These effects were evident in both autologous and allog eneic LTBMC systems. mAb S5 has been shown to enhance natural killer ( NK) activity, and more recent data indicate the increased cytoxic effe ct to be partially mediated through CD18. Thus, a possible role for mo dulation of the CD18 antigen in LTBMC by S5 was assessed by addition o f the anti-CD18 mAb 60.3. While the SS-induced reduction in total nona dherent cell production was not changed by addition of 60.3, the incre ase in progenitor cells stimulated by S5 was abrogated. These findings suggest a role for anti-CD44 antibody in changes in the marrow microe nvironment that may be responsible for facilitation of donor engraftme nt and, at least in part, CD18 may be involved in this phenomenon. The establishment of this in vitro LTBMC model will enable the mechanism to be further dissected.