title: Murine antithymocyte globulin T-cell depletion is mediated predominantly by macrophages, but the Fas/FasL pathway selectively targets regulatory T cells.
abstract: BACKGROUND
  Thymoglobulin is a T-cell-depleting polyclonal rabbit anti-human thymocyte antibody used clinically for immunosuppression in solid organ and hematopoietic stem-cell transplantation. By using a surrogate rabbit anti-mouse thymocyte globulin (mATG), we previously demonstrated that murine regulatory and memory T cells are preferentially spared from mATG depletion in vivo. The current studies were designed to determine whether different effector mechanisms are involved in differential depletion of T-cell subsets by mATG.
  
  
  METHODS
  Complement-dependent cytotoxicity, antibody-dependent cellular cytotoxicity (ADCC), and apoptotic mechanisms of depletion by mATG were evaluated in vitro and in vivo.
  
  
  RESULTS
  In vitro, there was evidence of differential susceptibility of T-cell subsets by different effector mechanisms where naïve and CD4 effector memory T cells show reduced susceptibility to apoptosis, whereas regulatory T cells are less susceptible to mATG-mediated complement-dependent cytotoxicity and ADCC. However, mATG treatment of mice depleted of ADCC effector cell types (neutrophils, natural killer cells, or macrophages) or deficient in complement C5 or Fas demonstrated that mATG depletion of all T-cell subsets is mediated primarily by macrophages and that the role of neutrophils, natural killer cells, and complement is minimal in vivo. Interestingly, the Fas/FasL pathway does play a role in regulatory T-cell depletion, which is likely a result of increased basal expression of Fas on these cells.
  
  
  CONCLUSIONS
  These data suggest that macrophages deplete most T cells by mATG in mice, but regulatory T cells are also uniquely susceptible to mATG-mediated Fas-dependent depletion.
authors.affiliation: Department Immunology and Clinical Laboratory Sciences, Biologics R&D, Genzyme Corporation, Framingham, MA, USA.
authors.forename: Kathleen S
authors.initials: KS
authors.lastname: Neff
authors.forename: Susan M
authors.initials: SM
authors.lastname: Richards
authors.forename: John M
authors.initials: JM
authors.lastname: Williams
authors.forename: Richard D
authors.initials: RD
authors.lastname: Garman
authors.forename: Melanie C
authors.initials: MC
authors.lastname: Ruzek
mesh_terms: D000818:Animals; D000920:Antibody-Dependent Cell Cytotoxicity; D000961:Antilymphocyte Serum / Q000494:pharmacology*; D017209:Apoptosis; D003182:Complement C5 / Q000378:metabolism; D003602:Cytotoxicity, Immunologic; D053222:Fas Ligand Protein / Q000276:immunology*; D006801:Humans; D066298:In Vitro Techniques; D007694:Killer Cells, Natural / Q000276:immunology; D008212:Lymphocyte Depletion; D008264:Macrophages / Q000276:immunology; D051379:Mice; D009504:Neutrophils / Q000276:immunology; D011817:Rabbits; D015398:Signal Transduction / Q000276:immunology; D016176:T-Lymphocyte Subsets / Q000166:cytology / Q000276:immunology; D013601:T-Lymphocytes / Q000166:cytology / Q000276:immunology*; D050378:T-Lymphocytes, Regulatory / Q000166:cytology / Q000276:immunology; D019014:fas Receptor / Q000276:immunology*
chemical_list: D000961:Antilymphocyte Serum; D003182:Complement C5; D053222:Fas Ligand Protein; C491899:Fas protein, mouse; C505658:Fasl protein, mouse; D019014:fas Receptor