Microchimerism is the presence of cells in an individual that have originated from another individual. The most common form of microchimerism is fetomaternal microchimerism, i.e., cells from a fetus pass through the placenta and establish cell lineages within the mother. Microchimerism was also described after transplantation of human organs in human recipients. Consequently, microchimerism may also be expected in xenotransplantation using pig cells or organs. Indeed, microchimerism was described in patients after xenotransplantations as well as in non-human primates after transplantation of pig organs. Here for the first time a comprehensive review of microchimerism in xenotransplantation is given. Since pig cells contain porcine endogenous retroviruses (PERVs) in their genome, detection of proviral DNA in the transplant recipients may be misinterpreted as infection of the recipient with PERV. To prevent this, methods discriminating between infection and microchimerism are described. This knowledge will be important for the interpretation of screening results in forthcoming human xenotransplantations.

Xenotransplantation is, like allotransplantation, usually associated with microchimerism, e.g., the presence of cells from the donor in the recipient. Microchimerism was reported in first xenotransplantation trials in humans as well as in most preclinical trials in non-human primates (for review see Denner, Viruses 2023, 15, 190). When using pigs as xenotransplantation donors, their cells contain porcine endogenous retroviruses (PERVs) in their genome. That makes it difficult to discriminate between microchimerism and PERV infection of the recipient. Here, we demonstrate which virological methods should be used to identify microchimerism, first of all screening for porcine cellular genes. Using porcine short interspersed nuclear sequences (SINES), which have hundred thousands of copies in the pig genome significantly increased the sensitivity of the screening for pig cells. Second, absence of PERV RNA demonstrated an absence of viral genomic RNA or expression as mRNA. Finally, absence of antibodies against PERV proteins conclusively demonstrated an absence of a PERV infection. When applying these methods for analyzing baboons after pig heart transplantation, microchimerism could be demonstrated in all animals. These methods can be used in future clinical trials.

The identification of fetal cells in the blood and tissues of healthy mothers raised questions about the potential alloreactive origins of various diseases, such as autoimmunity, diabetes, arteriosclerosis, and cancer. However, the extent to which alloreactivity contributes to these diseases remains unverified. This study presents a basic mathematical disease model based on the genes’ origin and immunological interactions. This model, grounded in set theory, classifies these immunological interactions based on their intrinsic, extrinsic, and allogeneic nature. Therefore, this model offers an unexpected common theoretical foundation for understanding alloreactivity, genetic diseases, and infections. Furthermore, this model provides a valuable framework for scrutinizing the role of alloreactivity in health and disease.

Endometriosis is an oestrogen-dependant reproductive disease, with genetic, vascular, neural, inflammatory and auto-immune characteristics. There are many theories about the etiology of endometriosis, however, all of these theories have limitations and do not explain all the locations that endometriosis is found or types of patients with endometriosis. The objective of this paper is to postulate the hypothesis that endometriosis is caused by Maternal Microchimerism, the presence of maternal cells in the fetus. A literature review was conducted, analysing the characteristics, current etiological theories of endometriosis, theory limitations and relationship of maternal microchimerism and endometriosis. At time of writing, there was no literature on maternal microchimerism and endometriosis. These results suggest that Maternal Microchimerism could be a cause of endometriosis. This could account for the genetic and auto-immune characteristics seen in people with endometriosis, inducing a micro-environment for vascular, neural and epigenetic changes. This could also account for account for endometriosis seen in non-menstruating patients, such as men, fetuses and post-menopausal women and endometriosis found in non-peritoneal locations. If the hypothesis of Maternal Microchimerism is correct, endometriosis could be considered a pregnancy-related disease that could affect all humans, changing the accepted demographics of patients and potentially new diagnostic techniques and treatment options for patients with endometriosis. Further studies are needed to test this hypothesis.