| Summary: | Limited persistence of xenogenic cells in human-mouse interspecies chimeras is an essential obstacle to overcome for the creation of disease models which allow the study of in vivo human cells. The two main barriers to donor cell survival are (1) having the capacity to respond appropriately to the xeno-environment and (2) competing with endogenous host cells.
This works uses human-mouse neural crest chimeras, generated by E8.5 in utero injection of human ESC/iPSC-derived neural crest cells (NCCs) into the gastrulating mouse embryo, as an experimental platform to better understand the limits of the barriers restricting interspecies chimerism. Here we present approaches which focus on recapitulating the melanocyte neural crest lineage to pave the wave for a novel immune-competent melanoma model:
First, we try adapting human donor cells to the mouse host environment via expression of a single mouse receptor (c-Kit) on human donor cells to rescue an evolutionarily divergent ligand/receptor interaction required for melanoblast proliferation and survival. We find that this extends the persistence of human NCCs, but does not lead to postnatal survival, suggesting c-Kit is not sufficient to rescue all aspects of melanocyte biology. In our second approach, we try to improve donor cell postnatal survival by combining proliferative advantage and pre-lineage biasing. We do this by injecting a genetically-defined human melanoma derived in primary melanocytes. We find they have the capacity to migrate in utero like primary mouse NCCs and contribute long-lasting donor cells in the dermis in post-natal chimeras. However, the immune-competent mouse hosts are not tolerized to human antigens. In the final approach, we address xenograft rejection as the final barrier to long-lasting interspecies chimerism. To explore whether central tolerance via human NC-derivative contribution to the chimera thymus is feasible, we develop fetal thymic organ culture (FTOC) co-cultures with human NCCs. We find that human NCCs and NCC-mesenchyme have a unique capacity to efficiently engraft onto mouse thymus ex vivo and preliminary evidence suggesting partial central tolerance is possible.
The success of these approaches ultimately suggests an updated framework for understanding how human-mouse chimerism barriers fit within the Developmental Hourglass Model and inform how future human-mouse interspecies chimeras may overcome these barriers within immune-competent hosts.
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