Extrafollicular CD4⁺ T and B Interaction Induces Chronic Gvhd in the Absence of Germinal Center Formation

Chronic graft-versus-host disease (GVHD) in patients with allogeneic hematopoietic cell transplantation (HCT) is an immune-mediated syndrome. Both CD4⁺ T and B cells are required for disease induction, but details of this interaction have not been fully defined. Other groups have reported that germinal centers are enlarged in mice with chronic GVHD and that GC formation is required for induction of the disease. On the other hand, patients with chronic GVHD have a predominance of IgG1 in the serum with little evidence of IgG somatic hypermutation by one year after HCT, suggesting a lack of GC formation (Suzuki et al: Blood. 1996;87:1873-1880; Glas et al: Blood. 2000; 96:1064-1069). Consistently, we observed that GCs were not detectable in mice with chronic GVHD, including MHC-matched C57BL/6 (H-2^b) recipients given LP/J (H-2^b) grafts and BALB/c (H-2^d) recipients given DBA/2 (H-2^d) or B10D2 (H-2^d) grafts, and MHC-mismatched BALB/c (H-2^d) recipients given C57BL/6 (H-2^b) grafts. Furthermore, BALB/c recipients given grafts from BCL6-deficient (BCL6 fl/fl Mb1-Cre) C57BL/6 (H-2^b) donors lacking the ability to form GC developed chronic GVHD with similar severity to recipients given control BCL6-sufficient grafts. Consistent with lack of GC formation, recipients of BCL6-deficient grafts had profound reduction of serum IgG2b and IgG2c but moderate reduction of IgG1 as compared to recipients given control BCL6-sufficient grafts. The presence of IgG1 indicates extrafollicular CD4⁺ T and B interaction, which is Stat-3 dependent. Further experiments showed BALB/c recipients given grafts from Stat3-deficient (Stat-3 fl/fl CD4-Cre) C57BL/6 donors did not develop chronic GVHD. Taken together, these results indicate that GC formation is not required for development of chronic GVHD and that extrafollicular CD4⁺ T and B interaction can induce chronic GVHD. This work is supported by NIH R01 AI066008 and The Beckman Research Institute Excellence Award (to D. Zeng).