Modification of host dendritic cells by microchimerism-derived extracellular vesicles generates split tolerance

Abstract

Maternal microchimerism (MMc) has been associated with development of allospecific transplant tolerance, antitumor immunity, and cross-generational reproductive fitness, but its mode of action is unknown. We found in a murine model that MMc caused exposure to the noninherited maternal antigens in all offspring, but in some, MMc magnitude was enough to cause membrane alloantigen acquisition (mAAQ; "cross-dressing") of host dendritic cells (DCs). Extracellular vesicle (EV)-enriched serum fractions from mAAQ + , but not from non-mAAQ, mice reproduced the DC crossdressing phenomenon in vitro. In vivo, mAAQ was associated with increased expression of immune modulators PD-L1 (programmed death-ligand 1) and CD86 by myeloid DCs (mDCs) and decreased presentation of allopeptide+self-MHC complexes, along with increased PD-L1, on plasmacytoid DCs (pDCs). Remarkably, both serum EV-enriched fractions and membrane microdomains containing the acquired MHC alloantigens included CD86, but completely excluded PD-L1. In contrast, EV-enriched fractions and microdomains containing allopeptide+self-MHC did not exclude PD-L1. Adoptive transfer of allospecific transgenic CD4 T cells revealed a "split tolerance" status in mAAQ + mice: T cells recognizing intact acquired MHC alloantigens proliferated, whereas those responding to allopeptide+self-MHC did not. Using isolated pDCs and mDCs for in vitro culture with allopeptide+self-MHC-specific CD4 T cells, we could replicate their normal activation in non-mAAQ mice, and PD-L1-dependent anergy in mAAQ + hosts. We propose that EVs provide a physiologic link between microchimerism and split tolerance, with implications for tumor immunity, transplantation, autoimmunity, and reproductive success.

Microchimerism: Defining and redefining the prepregnancy context – A review

“…The natural consequences of durable tolerance to NIMA on reproduction remain incompletely understood. However, tolerance to NIMA has been shown to be relevant to transplantation outcomes [23,24], dependent on persistence of MMc in offspring [12] and possibly related to MMc-derived extracellular vesicles [25], and likely relevant to next-generation pregnancy outcomes [26]. …”

Donor-derived exosomes

“…When the backcross breeding was done in such a way that maternal cells could be tagged by GFP, we found that higher levels of MMc were associated with 1) tolerance to a subsequent DBA-2 heart allograft; 2) the presence of immune regulatory cells specific for the NIMA d Ag in spleen and lymph nodes; and 3) the presence of GFP + maternal cells in CD11b + and CD11c + myeloid cell populations [45]. Since CD11c + DCs are also known to be capable of secreting large numbers of EVs/exosomes, we tested whether the serum of the offspring contained MHC Ags in the form of EVs [46]. We found that we could divide the NIMA d -exposed offspring into 2 types – one type, with low levels of MMc, had no membrane-bound alloantigen acquired on the surface of their mDCs (hereafter referred to as “non-mAAQ” mice).…”

“…In mice with membrane alloantigen acquisition (mAAQ + ), by contrast, the IE molecule was present both in the EV-free serum fraction and the ultracentrifuge pellet containing EVs. Analyses of EV-enriched fractions showed the presence of exosome-sized vesicles (50–110 nm diameter), enriched for the exosome marker CD9, with low content of Golgi, endoplasmic reticulum, and histone proteins [46]. Furthermore, when EVs from the mAAQ + serum were tested in a short-term culture with B6 (H2 d-negative ) cells, both plasmacytoid DCs (pDCs) and mDCs were found to acquire surface H2-K d (MHC class-I) and IA d (MHC class-II) molecules of maternal origin.…”

“…Remarkably, TEa cells only proliferated in the non-mAAQ mice. In contrast, transfer into the mAAQ + mice caused TEa cells to undergo abortive activation, followed by anergy [46]. …”

Exosomes

“…The recipient APCs cross-dressed with donor–derived exosomes stimulate proliferation and terminal differentiation of directly allo-reactive T cells through the semi-direct pathway of allo-recognition 4,5 . Transfer via exosomes of noninherited maternal MHC allo-Ag (NIMAs) from maternal microchimeric cells to host APCs is behind the T cell split tolerance against NIMAs 6 .…”