Non-classical MHC-E (Mamu-E) expression in the rhesus monkey placenta

In vitro differentiation of macaque extravillous trophoblasts in a low oxygen environment

INTRODUCTION

Early primate placental development occurs within a low oxygen (O2) environment, whereas in vitro differentiation of trophoblasts is performed at supraphysiologic O2 levels. Macaque trophoblast stem cells (TSCs) are capable of differentiation to extravillous trophoblasts (EVTs) in vitro, yet the morphological heterogeneity observed across cells lines necessitates evaluation of optimal culture conditions. Our objectives were to determine the impact of oxygen on the in vitro differentiation of macaque TSCs and to refine the molecular characterization of TSC-differentiated EVTs.

METHODS

Macaque TSCs were differentiated to EVTs in either 20% or 5% O2. Gene and protein expression profiles were compared between TSCs and EVTs and between differentiation conditions. Immunohistochemical analysis was performed on early gestation macaque placental tissues to assess in vivo expression of Ki-67, NCAM1 and monkey chorionic gonadotropin (mCG).

RESULTS

EVTs differentiated in 20% O2 had significantly higher expression of CGA, CGB and NOTCH2 and decreased HIF1A expression compared to those cultured in 5% O2. Regardless of oxygen condition, nearly all EVTs expressed NCAM1 and Mamu-AG, the macaque-specific homolog of human EVT marker HLA-G. In vivo placental expression of NCAM1 was restricted to EVTs within the trophoblastic shell and endovasculature, revealing a macaque EVT marker within the placenta.

DISCUSSION

Reduced oxygen minimally impacted macaque EVT differentiation in vitro. Elevated expression of the endovascular EVT marker NOTCH2 potentially suggests that 20% O2 supported differentiation of more mature EVTs. Altogether, a standard 20% O2 environment supports macaque EVT differentiation in vitro and the results further validate the identity of macaque TSC-differentiated EVTs.

KIR3DL05 and KIR3DS02 Recognition of a Nonclassical MHC Class I Molecule in the Rhesus Macaque Implicated in Pregnancy Success

Knowledge of the MHC class I ligands of rhesus macaque killer-cell Ig-like receptors (KIRs) is fundamental to understanding the role of natural killer (NK) cells in this species as a nonhuman primate model for infectious diseases, transplantation and reproductive biology. We previously identified Mamu-AG as a ligand for KIR3DL05. Mamu-AG is a nonclassical MHC class I molecule that is expressed at the maternal-fetal interface of the placenta in rhesus macaques similar to HLA-G in humans. Although Mamu-AG and HLA-G share similar molecular features, including limited polymorphism and a short cytoplasmic tail, Mamu-AG is considerably more polymorphic. To determine which allotypes of Mamu-AG serve as ligands for KIR3DL05, we tested reporter cell lines expressing five different alleles of KIR3DL05 (KIR3DL05*001, KIR3DL05*004, KIR3DL05*005, KIR3DL05*008 and KIR3DL05*X) for responses to target cells expressing eight different alleles of Mamu-AG. All five allotypes of KIR3DL05 responded to Mamu-AG2*01:01, two exhibited dominant responses to Mamu-AG1*05:01, and three had low but detectable responses to Mamu-AG3*03:01, -AG3*03:02, -AG3*03:03 and -AG3*03:04. Since KIR3DL05*X is the product of recombination between KIR3DL05 and KIR3DS02, we also tested an allotype of KIR3DS02 (KIR3DS02*004) and found that this activating KIR also recognizes Mamu-AG2*01:01. Additional analysis of Mamu-AG variants with single amino acid substitutions identified residues in the α1-domain essential for recognition by KIR3DL05. These results reveal variation in KIR3DL05 and KIR3DS02 responses to Mamu-AG and define Mamu-AG polymorphisms that differentially affect KIR recognition.

Placenta-derived macaque trophoblast stem cells: differentiation to syncytiotrophoblasts and extravillous trophoblasts reveals phenotypic reprogramming

Nonhuman primates are excellent models for studying human placentation as experimental manipulations in vitro can be translated to in vivo pregnancy. Our objective was to develop macaque trophoblast stem cells (TSCs) as an in vitro platform for future assessment of primate trophoblast development and function. Macaque TSC lines were generated by isolating first and second trimester placental villous cytotrophoblasts followed by culture in TSC medium to maintain cellular proliferation. TSCs grew as mononuclear colonies, whereas upon induction of syncytiotrophoblast (ST) differentiation multinuclear structures appeared, indicative of syncytium formation. Chorionic gonadotropin secretion was > 4000-fold higher in ST culture media compared to TSC media. The secretion of chorionic gonadotropin by TSC-derived ST reflects a reprogramming of macaque TSCs to an earlier pregnancy phenotype. Characteristic trophoblast hallmarks were defined in TSCs and ST including expression of C19MC miRNAs and the macaque placental nonclassical MHC class I molecule, Mamu-AG. Extravillous trophoblasts (EVTs) were derived that express macaque EVT markers Mamu-AG and CD56, and also secrete high levels of MMP2. Our analyses of macaque TSCs suggests that these cells represent a proliferative, self-renewing population capable of differentiating to STs and EVTs in vitro thereby establishing an experimental model of primate placentation.

HLA-E: exploiting pathogen-host interactions for vaccine development

Viruses, when used as vectors for vaccine antigen delivery, can induce strong cellular and humoral responses against target epitopes. Recent work by Hansen et al. describes the use of a cytomegalovirus‐vectored vaccine, which is able to generate a stable effector‐memory T cell population at the sites of vaccination in rhesus macaques. This vaccine, targeted towards multiple epitopes in simian immunodeficiency virus (SIV), did not induce classical CD8⁺ T cells. However, non‐canonical CD8⁺ T cell induction occurred via major histocompatibility complex (MHC) class II and MHC‐E. The MHC‐E‐restricted T cells could recognize broad epitopes across the SIV peptides, and conferred protection against viral challenge to 55% of vaccinated macaques. The human homologue, human leucocyte antigen (HLA)‐E, is now being targeted as a new avenue for vaccine development. In humans, HLA‐E is an unusually oligomorphic class Ib MHC molecule, in comparison to highly polymorphic MHC class Ia. Whereas MHC class Ia presents peptides derived from pathogens to T cells, HLA‐E classically binds defined leader peptides from class Ia MHC peptides and down‐regulates NK cell cytolytic activity when presented on the cell surface. HLA‐E can also restrict non‐canonical CD8⁺ T cells during natural infection with various pathogens, although the extent to which they are involved in pathogen control is mostly unknown. In this review, an overview is provided of HLA‐E and its ability to interact with NK cells and non‐canonical T cells. Also discussed are the unforeseen beneficial effects of vaccination, including trained immunity of NK cells from bacille Calmette–Guérin (BCG) vaccination, and the broad restriction of non‐canonical CD8⁺ T cells by cytomegalovirus (CMV)‐vectored vaccines in pre‐clinical trials.

The immunology of the macaque placenta: A detailed analysis and critical comparison with the human placenta

The cynomolgus monkey is increasingly considered in toxicological research as the most appropriate model for humans due to the species' close physiological contiguity, including reproductive physiology. Here, literature on the cynomolgus monkey placenta is reviewed in regards to its similarity to the human placenta and particularly for its immunological role, which is not entirely mirrored in humans. Pertinent original data are included in this article. The cynomolgus monkey placenta is evaluated based on three aspects: first, morphological development; second, the spatial and temporal appearance of maternal and fetal immune cells and certain immune cell products of the innate and adaptive immune systems; and third, the expression of relevant immune tolerance-related molecules including the homologs of anti-human leucocyte antigen, indoleamine 2,3-dioxygenase, FAS/FAS-L, annexin II, and progesterone. Parameters relevant to the immunological role of the placenta are evaluated from the immunologically immature stage of gestational day (GD) 50 until more mature stages close to birth. Selected comparisons are drawn with human and other laboratory animal placentas. In conclusion, the cynomolgus monkey placenta has a high degree of morphological and physiological similarity to the human placenta. However, there are differences in the topographical distribution of cell types and immune tolerance-related molecules. Three basic features are recognized: (1) the immunological capacity of the placenta changes throughout the lifetime of the organ; (2) these immunological changes include multiple parameters such as morphological adaptations, cell type involvement, and changes in immune-relevant molecule expression; and (3) the immune systems of two genetically disparate individuals (mother and child) are functionally intertwined at the maternal-fetal interface.

The Role of MHC-E in T Cell Immunity Is Conserved among Humans, Rhesus Macaques, and Cynomolgus Macaques

MHC-E is a highly conserved nonclassical MHC class Ib molecule that predominantly binds and presents MHC class Ia leader sequence-derived peptides for NK cell regulation. However, MHC-E also binds pathogen-derived peptide Ags for presentation to CD8+ T cells. Given this role in adaptive immunity and its highly monomorphic nature in the human population, HLA-E is an attractive target for novel vaccine and immunotherapeutic modalities. Development of HLA-E-targeted therapies will require a physiologically relevant animal model that recapitulates HLA-E-restricted T cell biology. In this study, we investigated MHC-E immunobiology in two common nonhuman primate species, Indian-origin rhesus macaques (RM) and Mauritian-origin cynomolgus macaques (MCM). Compared to humans and MCM, RM expressed a greater number of MHC-E alleles at both the population and individual level. Despite this difference, human, RM, and MCM MHC-E molecules were expressed at similar levels across immune cell subsets, equivalently upregulated by viral pathogens, and bound and presented identical peptides to CD8+ T cells. Indeed, SIV-specific, Mamu-E-restricted CD8+ T cells from RM recognized antigenic peptides presented by all MHC-E molecules tested, including cross-species recognition of human and MCM SIV-infected CD4+ T cells. Thus, MHC-E is functionally conserved among humans, RM, and MCM, and both RM and MCM represent physiologically relevant animal models of HLA-E-restricted T cell immunobiology.

On the role of placental Major Histocompatibility Complex and decidual leukocytes in implantation and pregnancy success using non-human primate models

While there is broad agreement that interactions of the human maternal immune system with the tissues and cells of the implanting embryo are likely to be critical contributors to pregnancy success, there remains a dearth of information which directly confirms this expectation. Although animal models of reproductive function often provide opportunities for confirming such hypotheses, progress in this area has been sporadic due to limitations of traditional laboratory or agricultural animal models, such as rodents, sheep, pigs and cattle. Many of these limitations derive from divergent modes of implantation and placentation across mammalian species. Over the past decade there has been progress in the development of the nonhuman primate as a model in which to address questions of pregnancy success in the area of immunology. The purpose of this review is to compare available model species, summarize current knowledge and recent progress with an emphasis on experimental in vivo manipulations, and suggest areas available for additional study and growth.