title: Down-Regulation of Surface CD28 under Belatacept Treatment: An Escape Mechanism for Antigen-Reactive T-Cells.
abstract: BACKGROUND
  The co-stimulatory inhibitor of the CD28-CD80/86-pathway, belatacept, allows calcineurin-inhibitor-free immunosuppression in kidney transplantation. However, aggressive T-cell mediated allogeneic responses have been observed in belatacept-treated patients, which could be explained by effector-memory T-cells that lack membrane expression of CD28, i.e. CD28-negative (CD28NULL) T-cells. CD28-positive (CD28POS) T-cells that down regulate their surface CD28 after allogeneic stimulation could also pose a threat against the renal graft. The aim of this study was to investigate this potential escape mechanism for CD28POS T-cells under belatacept treatment.
  
  
  METHODS
  PBMCs, isolated T-cell memory subsets and isolated CD28POS T-cells were obtained from end-stage renal disease (ESRD) patients and co-cultured with allo-antigen in the presence of belatacept to mimic allogeneic reactions in kidney-transplant patients under belatacept treatment. As a control, IgG was used in the absence of belatacept.
  
  
  RESULTS
  Despite high in vitro belatacept concentrations, a residual T-cell growth of ±30% was observed compared to the IgG control after allogeneic stimulation. Of the alloreactive T-cells, the majority expressed an effector-memory phenotype. This predominance for effector-memory T-cells within the proliferated cells was even larger when a higher dose of belatacept was added. Contrary to isolated naïve and central-memory T cells, isolated effector-memory T cells could not be inhibited by belatacept in differentiation or allogeneic IFNγ production. The proportion of CD28-positive T cells was lower within the proliferated T cell population, but was still substantial. A fair number of the isolated initially CD28POS T-cells differentiated into CD28NULL T-cells, which made them not targetable by belatacept. These induced CD28NULL T-cells were not anergic as they produced high amounts of IFNγ upon allogeneic stimulation. The majority of the proliferated isolated originally CD28POS T-cells, however, still expressed CD28 and also expressed IFNγ.
  
  
  CONCLUSIONS
  This study provides evidence that, apart from CD28NULL T-cells, also CD28POS, mostly effector-memory T-cells can mediate allogeneic responses despite belatacept treatment.
authors.affiliation: Department of Internal Medicine, Section of Transplantation and Nephrology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Gretchen N
authors.initials: GN
authors.lastname: de Graav
authors.affiliation: Department of Internal Medicine, Section of Transplantation and Nephrology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Hesselink
authors.affiliation: Department of Internal Medicine, Section of Transplantation and Nephrology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Marjolein
authors.initials: M
authors.lastname: Dieterich
authors.affiliation: Department of Internal Medicine, Section of Transplantation and Nephrology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Rens
authors.initials: R
authors.lastname: Kraaijeveld
authors.affiliation: Department of Internal Medicine, Section of Transplantation and Nephrology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Willem
authors.initials: W
authors.lastname: Weimar
authors.affiliation: Department of Internal Medicine, Section of Transplantation and Nephrology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands.
authors.forename: Carla C
authors.initials: CC
authors.lastname: Baan
mesh_terms: D000069594:Abatacept / Q000008:administration & dosage / Q000494:pharmacology*; D064591:Allografts; D018106:CD28 Antigens / Q000378:metabolism*; D002454:Cell Differentiation / Q000187:drug effects; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D015536:Down-Regulation / Q000187:drug effects; D006801:Humans; D007156:Immunologic Memory / Q000187:drug effects; D016130:Immunophenotyping; D007166:Immunosuppressive Agents / Q000008:administration & dosage / Q000494:pharmacology*; D007371:Interferon-gamma / Q000096:biosynthesis; D007519:Isoantigens / Q000008:administration & dosage; D007676:Kidney Failure, Chronic / Q000276:immunology / Q000628:therapy; D016030:Kidney Transplantation; D016176:T-Lymphocyte Subsets / Q000166:cytology / Q000187:drug effects* / Q000276:immunology*
chemical_list: D018106:CD28 Antigens; D007166:Immunosuppressive Agents; D007519:Isoantigens; D000069594:Abatacept; D007371:Interferon-gamma