ACTIVATION AND ROLE OF T REGULATORY CELLS

B. Fazekas de st groth¹, Y.W. Loh², D. Gracey³, B. Roediger⁴, S.I. Alexander^5
1T Cell Biology Research Program, Centenary Institute, Newtown/NSW/AUSTRALIA, ²T Cell Biology Research Program, Centenary Institute, Locked Bag #6/NSW/AUSTRALIA, ³Renal Medicine, Royal Prince Alfred Hospital, Camperdown/NSW/AUSTRALIA, ⁴Immune Imaging Research Program, Centenary Institute, Newtown/NSW/AUSTRALIA, ⁵Nephrology, The Children's Hospital at Westmead, Westmead/AUSTRALIA

Body: Introduction: The role of naïve versus heterologously primed T cells in graft rejection remains controversial. Purified populations of naïve T cell receptor (TCR) transgenic cells can reject skin grafts after transfer to immunodeficient hosts, but this could result from activation via spontaneous proliferation in the lymphopenic environment. Here we examined skin and islet graft rejection in a model in which 5C.C7 TCR transgenic T cells recognise IA^s, in addition to their primary specificity for moth cytochrome c (MCC) in the context of IE^k or IE^b. The important feature of this model is that all 5C.C7 T cells remain naïve in transgenic mice on a B10.BR/RAG^-/- background, and transferred naïve 5C.C7 /B10.BR/RAG^-/- cells do not undergo spontaneous proliferation in either RAG^-/- or wt hosts. Methods: Allogeneic I-A^s skin or islets from B10.S mice were transplanted onto B10.BR wildtype or 5C.C7/B10.BR/RAG^-/- recipients. Results: B10.S skin allografts on B10.BR mice were rejected with normal tempo (d7,9,10,11,12,12: mean 10.2±0.8), as were islets (11,16,16,17,19,43,65: mean 26.7±7.5). Rejection times were unaffected by adoptive transfer of a small cohort of 5C.C7 T cells. Unexpectedly, very little proliferation of naïve 5C.C7 T cells was seen in graft draining lymph nodes. We therefore tested whether graft DCs were able to migrate to the nodes. Skin from transgenic mice expressing GFP-tagged IA^b was grafted onto syngeneic H-2^b and allogeneic H-2^k hosts. No GFP-expressing DCs could be detected in draining LN in either host. As a positive control, subcutaneously injected DCs from GFP-tagged IA^b mice were easily detected in draining LN. As an additional control, 5C.C7/B10.BR/RAG^-/ cells responded vigorously to injected I-A^s DCs, but not to skin grafts. To test whether naïve alloreactive cells could reject grafts, we grafted 5C.C7/B10.BR/RAG^-/- mice with skin or islets from B10.S donors. B10.S skin (n=12) and islet (n=5) allografts in 5C.C7/B10.BR/RAG^-/- mice were all accepted indefinitely (>100 days). To rule out poor reactivity of 5C.C7/B10.BR/RAG^-/- cells to I-A^s, 5C.C7 cells from MCC-primed 5C.C7/B10.BR/RAG^-/- donors were sorted into three populations: naïve (CD44^loCD62L^hi), effector memory (CD44^hiCD62L^lo) and central memory (CD44^hiCD62L^hi), prior to adoptive transfer into long-term 5C.C7/B10.BR/RAG^-/- skin graft recipients. Allografts remained intact for >100 additional days upon transfer of naïve cells (n=4) while mice rejected grafts 13,14,14,16 and 22d after effector memory cell transfer and 27,52, and >87d after central memory cell transfer. Conclusion: These data suggest that naïve allograft-specific CD4 T cells cannot initiate graft rejection, because viable graft dendritic cells do not migrate to graft draining nodes. In contrast, allo-crossreactive memory cells primed by environmental exposure before grafting are uniquely capable of initiating graft rejection by entering the graft directly from the bloodstream.

Disclosure: All authors have declared no conflicts of interest.