Peripheral tissues reprogram CD8+ T cells for pathogenicity during graft-versus-host disease

Graft-versus-host disease (GVHD) is a life-threatening complication of allogeneic stem cell transplantation induced by the influx of donor-derived effector T cells (TE) into peripheral tissues. Current treatment strategies rely on targeting systemic T cells; however, the precise location and nature of instructions that program TE to become pathogenic and trigger injury are unknown. We therefore used weighted gene coexpression network analysis to construct an unbiased spatial map of TE differentiation during the evolution of GVHD and identified wide variation in effector programs in mice and humans according to location. Idiosyncrasy of effector programming in affected organs did not result from variation in T cell receptor repertoire or the selection of optimally activated TE. Instead, TE were reprogrammed by tissue-autonomous mechanisms in target organs for site-specific proinflammatory functions that were highly divergent from those primed in lymph nodes. In the skin, we combined the correlation-based network with a module-based differential expression analysis and showed that Langerhans cells provided in situ instructions for a Notch-dependent T cell gene cluster critical for triggering local injury. Thus, the principal determinant of TE pathogenicity in GVHD is the final destination, highlighting the need for target organ-specific approaches to block immunopathology while avoiding global immune suppression.

Depletion of CD11c⁺ cells in the CD11c.DTR model drives expansion of unique CD64⁺ Ly6C⁺ monocytes that are poised to release TNF-α

Dendritic cells (DCs) play a vital role in innate and adaptive immunities. Inducible depletion of CD11c(+) DCs engineered to express a high-affinity diphtheria toxin receptor has been a powerful tool to dissect DC function in vivo. However, despite reports showing that loss of DCs induces transient monocytosis, the monocyte population that emerges and the potential impact of monocytes on studies of DC function have not been investigated. We found that depletion of CD11c(+) cells from CD11c.DTR mice induced the expansion of a variant CD64(+) Ly6C(+) monocyte population in the spleen and blood that was distinct from conventional monocytes. Expansion of CD64(+) Ly6C(+) monocytes was independent of mobilization from the BM via CCR2 but required the cytokine, G-CSF. Indeed, this population was also expanded upon exposure to exogenous G-CSF in the absence of DC depletion. CD64(+) Ly6C(+) monocytes were characterized by upregulation of innate signaling apparatus despite the absence of inflammation, and an increased capacity to produce TNF-α following LPS stimulation. Thus, depletion of CD11c(+) cells induces expansion of a unique CD64(+) Ly6C(+) monocyte population poised to synthesize TNF-α. This finding will require consideration in experiments using depletion strategies to test the role of CD11c(+) DCs in immunity.

Evaluation of the medial soft-tissue restraints of the extensor mechanism of the knee

We performed an anatomical dissection of the medial soft-tissue retinacular fibers that restrain lateral patellar displacement and found that the medial patellofemoral ligament inserts not only on the patella but also on the undersurface of the distal aspect of the quadriceps mechanism. The deep capsular layer contained substantial retinacular fibers that were associated with the medial patellomeniscal ligament. Functional studies of the relative contributions of the medial soft-tissue restraints in the prevention of lateral patellar displacement were also performed. Twenty-five fresh-frozen specimens of the knee, obtained after amputations (nineteen specimens) or from cadavera (six specimens) were tested biomechanically on a universal testing instrument. We ranked the soft-tissue restraints, in order of their relative contributions to the restraining force, on the basis of the percentage of force provided by the retinacular and ligamentous tissue that resisted the lateral displacement of the patella. The medial patellofemoral ligament, although varying in size and importance, was found to be the major medial soft-tissue restraint that prevented lateral displacement of the distal knee-extensor mechanism, contributing an average of 53 per cent of the total force. The patellomeniscal ligament and associated retinacular fibers in the deep capsular layer of the knee, which were previously thought to be functionally unimportant in the stabilization of the patella, contributed an average of 22 per cent of the total force. The previously described retinacular fibers (the patellotibial band) were functionally unimportant in the prevention of lateral displacement.