Autoimmunity versus allo- and xeno-reactivity in SCID mice

Efficient peripheral construction of functional human regulatory CD4(+)CD25(high)Foxp3(+) T cells in NOD/SCID mice grafted with fetal human thymus/liver tissues and CD34(+) cells

Regulatory T cells, especially CD4(+)CD25(+) regulatory T cells are critical regulators of immune tolerance in humans and mice. Mice with humanized immunity have been developed by various transplantation strategies of human tissues or cells related to immunity, which are being extensively applied in biomedical research. However, it is unclear whether human CD4(+)CD25(+) regulatory T cells can normally develop in human thymic grafts and efficiently populate in the periphery in NOD/SCID mouse recipients. In human thymic grafts, high percentage of mature human CD4(+)CD25(high) regulatory T cells was detected. Human CD4(+)CD25(+) regulatory T cells maturing in fetal human thymus grafts could subsequently output to the periphery of NOD/SCID mouse recipients. Importantly, these cells exhibited Foxp3(+)CD45RO(+)CTLA4(+)CD127(-) phenotype, similarly to those in healthy individuals. In addition, human CD4(+)CD25(+) regulatory T cells maturing in human thymic grafts suppressed proliferative response of CD4(+)CD25(-) T cells to allogeneic antigens, though the peripheral CD4(+)CD25(+) regulatory T cells in fetal human thymus-grafted NOD/SCID mice showed somewhat decreased immunosuppressive ability compared with normal CD4(+)CD25(+) regulatory T cells. Thus, this humanized animal model is suitable for examining development and function of human CD4(+)CD25(+) regulatory T cells in vivo.

Mouse models with human immunity and their application in biomedical research

Biomedical research in human beings is largely restricted to in vitro studies that lack complexity of a living organism. To overcome this limitation, humanized mouse models are developed based on immunodeficient characteristics of severe combined immunodeficiency (SCID) or recombination activating gene (Rag)(null) mice, which can accept xenografts. Peripheral constitution of human immunity in SCID or Rag(null) mice has been achieved by transplantation of mature human immune cells, foetal human thymus, bone marrow, liver tissues, lymph nodes or a combination of these, although efficiency needs to be improved. These mouse models with constituted human immunity (defined as humanized mice in the present text) have been widely used to investigate the basic principles of human immunobiology as well as complex pathomechanisms and potential therapies of human diseases. Here, elements of an ideal humanized mouse model are highlighted including genetic and non-genetic modification of recipient mice, transplantation strategies and proposals to improve engraftments. The applications of the humanized mice to study the development and response of human immune cells, human autoimmune diseases, virus infections, transplantation biology and tumour biology are reviewed as well.