Marilen Benner

SUMMARY AND DISCUSSION 225 8 and undetected pathologic cause. For example, we do not have data on whether the samples were collected at the end of a cycle during which unprotected intercourse took place. Exposure to seminal fluid or even failed implantation could affect the lymphocyte composition observed in menstrual blood-derived lymphocytes (52-54). Moreover, fertility status was confirmed for about half of the participating women, as they gave birth in the past. Thus, while we made the assumption that the majority of women was able to conceive, it might be possible that for some this was not the case and consequently women with aberrant immunological profile were included. We investigated in how far menstrual blood can identify an unfavorable immunological environment connected to RPL in Chapter 5 . The decidualized endometrium forms the “fertile ground” for a healthy placenta; successful crosstalk of the blastocyst and the decidua determines if correct placenta formation can be initiated (55). Immune alterations can impact the trophoblast invasion, and endometrial biopsies showed a reduction in NK bright and CD8 + T cell frequencies compared to healthy controls (56). However, the number and extent of these data is limited, and establishing an immunological profile of the endometrium of healthy women is challenging when relying on tissue from biopsies. Not only the invasive nature of the procedure, and thus ethical concerns, hamper sample collection, but the number of lymphocytes isolated from a single tissue sample limits the information that can be gathered from each volunteer. We were able to analyze more than 10 times more cells through non-invasive menstrual blood collection and cell isolation. This way, in Chapter 5 , we investigated the immunological landscape of the non-pregnant uterus in women suffering from recurrent pregnancy loss. The general immune profiling strategy as described in Chapter 2 was expanded with an additional investigation of B cell subsets based on our findings of Chapter 4. Using machine learning, we were able to define key immune subsets of either peripheral or menstrual blood that allow for correct classification. Thus, RPL is connected to altered immunity. Of note, B cell subsets contributed to cohort distinction for both sample types, emphasizing the possible value of considering B cells in future approaches. We included cytomegalovirus (CMV) seropositivity of our donors in the analysis. CMV causes lifelong latent infection and with, a prevalence of estimated 40 to 100 percent globally, affects a majority of women, especially with increasing age (57). While it rarely elicits symptoms in immunocompetent individuals, it persistently affects immunity (58). Immune evasionmechanisms of CMV are known to especially alter NK and T cell responses, as both are major components of the defense against the virus (59). As such, whether or not a woman has encountered CMV has to be considered a possible confounder. Primary CMV infection during pregnancy is the most frequent cause for congenital disease in developed countries, but it is unclear whether latent CMV infection also deserves a negative connotation with regard to healthy pregnancy. CMV has coevolved with human development for millions of years. Hampering reproduction of its host would negatively impact persistence of the virus. The data presented in Chapter 5 show that