AUTHOR=Alsughayyir Jawaher , Chhabra Manu , Qureshi M. Saeed , Mallik Mekhola , Ali Jason M. , Gamper Ivonne , Moseley Ellen L. , Peacock Sarah , Kosmoliaptsis Vasilis , Goddard Martin J. , Linterman Michelle A. , Motallebzadeh Reza , Pettigrew Gavin J. 

TITLE=Relative Frequencies of Alloantigen-Specific Helper CD4 T Cells and B Cells Determine Mode of Antibody-Mediated Allograft Rejection

JOURNAL=Frontiers in Immunology

VOLUME=9

YEAR=2019

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2018.03039

DOI=10.3389/fimmu.2018.03039

ISSN=1664-3224

ABSTRACT=<p>Humoral alloimmunity is now recognized as a major determinant of transplant outcome. MHC glycoprotein is considered a typical T-dependent antigen, but the nature of the T cell alloresponse that underpins alloantibody generation remains poorly understood. Here, we examine how the relative frequencies of alloantigen-specific B cells and helper CD4 T cells influence the humoral alloimmune response and how this relates to antibody-mediated rejection (AMR). An MHC-mismatched murine model of cardiac AMR was developed, in which T cell help for alloantibody responses in T cell deficient (<italic>Tcrbd</italic><sup>−/−</sup>) C57BL/6 recipients against donor H-2K<sup>d</sup> MHC class I alloantigen was provided by adoptively transferred “TCR75” CD4 T cells that recognize processed H-2K<sup>d</sup> allopeptide via the indirect-pathway. Transfer of large numbers (5 × 10<sup>5</sup>) of TCR75 CD4 T cells was associated with rapid development of robust class-switched anti-H-2K<sup>d</sup> humoral alloimmunity and BALB/c heart grafts were rejected promptly (MST 9 days). Grafts were not rejected in T and B cell deficient <italic>Rag2</italic><sup>−/−</sup> recipients that were reconstituted with TCR75 CD4 T cells or in control (non-reconstituted) <italic>Tcrbd</italic><sup>−/−</sup> recipients, suggesting that the transferred TCR75 CD4 T cells were mediating graft rejection principally by providing help for effector alloantibody responses. In support, acutely rejecting BALB/c heart grafts exhibited hallmark features of acute AMR, with widespread complement C4d deposition, whereas cellular rejection was not evident. In addition, passive transfer of immune serum from rejecting mice to <italic>Rag2</italic><sup>−/−</sup> recipients resulted in eventual BALB/c heart allograft rejection (MST 20 days). Despite being long-lived, the alloantibody responses observed at rejection of the BALB/c heart grafts were predominantly generated by extrafollicular foci: splenic germinal center (GC) activity had not yet developed; IgG secreting cells were confined to the splenic red pulp and bridging channels; and, most convincingly, rapid graft rejection still occurred when recipients were reconstituted with similar numbers of <italic>Sh2d1a</italic><sup>−/−</sup> TCR75 CD4 T cells that are genetically incapable of providing T follicular helper cell function for generating GC alloimmunity. Similarly, alloantibody responses generated in <italic>Tcrbd</italic><sup>−/−</sup> recipients reconstituted with smaller number of wild-type TCR75 CD4 T cells (10<sup>3</sup>), although long-lasting, did not have a discernible extrafollicular component, and grafts were rejected much more slowly (MST 50 days). By modeling antibody responses to Hen Egg Lysozyme protein, we confirm that a high ratio of antigen-specific helper T cells to B cells favors development of the extrafollicular response, whereas GC activity is favored by a relatively high ratio of B cells. In summary, a relative abundance of helper CD4 T cells favors development of strong extrafollicular alloantibody responses that mediate acute humoral rejection, without requirement for GC activity.</p><p>This work is composed of two parts, of which this is Part I. Please read also Part II: Chhabra et al., 2019.</p>