Background

Costimulatory blockade plus donor-specific splenocyte transfusion (DST) induces long-term graft acceptance, however the mechanism by which this tolerance occurs remains incompletely defined. The lymph node (LN) contains a basic scaffolding structure comprised of collagens, laminins and stromal fibers such as ER-TR7. These fibers form a protein meshwork foundation for cell interactions and bind chemokines that guide cells to specific, instructive microdomains. This study tested the hypothesis that tolerance induction involves modification of lymph node structure and chemokine expression.

Methods

Tolerance was induced by treating C57BL/6 mice with BALB/c DST + anti-CD40L mAb, while immunity was induced by treating mice with DST only. Mice were euthanized 3 to 7 days post-treatment and quantitative immunohistochemistry of LN defined the amount and location of the structural elements collagen III, ER-TR7, desmin, laminin, the high endothelial venules (HEV), and the chemokines CCL19 and CCL21.

Results

LN in tolerant and immune mice displayed distinct structural modifications and chemokine expression. In the LN of tolerant mice, ER-TR7 increased and peaked at day 5 post-tolerance induction. Laminin decreased initially following tolerance induction, with expression increasing 5 days later. In contrast, ER-TR7 in the LN peaked 3 days following immunization, and laminin expression decreased to levels below that observed in naÏve LN. Following tolerance induction or immunization, an increase in both desmin and collagen III occurred. With respect to chemokine expression, CCL19 expression increased gradually after day 3 in tolerant mice. In contrast, CCL19 expression peaked at day 3 in immune mice. Interestingly, CCL21 expression decreased following both tolerization and immunity.

Conclusion

Tolerance induction results in structural changes within the LN. ER-TR7 and laminin are essential LN structural stromal fibers, and CCL19 is important for T cell and dendritic cell LN chemoattraction. Hence, these results suggest that tolerance induction leads to changes in both LN structure and function. These changes can subsequently affect T cell migration, homing and differentiation in the LN. Hence, this remodeling choreographs the encounters and interactions between antigen reactive cells and their cognate antigen resulting in tolerance as opposed to immunity. These findings suggest that the LN is a malleable structure, and changes in both physical structure and chemokine expression affect LN function.

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