Did you know that all of us harbor human cells that are not our own? I didn’t, until I saw a recent book review in Nature. Hidden Guests, by Lise Barnéoud, is the first popular science book about microchimerism, a phenomenon whereby cells from our biological relatives can persist in our bodies for decades. Microchimerism occurs because cells can pass between mother and fetus during pregnancy or between twins sharing a uterus. Although rare, these cells have been found in nearly all tissues and have been linked to both benefits (e.g., wound healing and immune tolerance) and harm (e.g., pregnancy complications and autoimmune disorders). However, our understanding of their impact remains limited for several reasons. Firstly, microchimeric cells are difficult to detect because of their rarity and genetic similarity to surrounding cells. To identify such cells, especially within tissues other than blood, researchers have traditionally relied on looking for Y chromosomes, limiting them to studying mothers who have given birth to sons. Microchimeric cells are also highly diverse, and tools to manipulate them are limited, making it difficult to advance the field beyond correlations and toward causation.
Two recent papers tackle these hurdles. In the first paper, Thomas Kroneis’s group used spatial transcriptomics and single-letter differences in the cells’ genetic code (SNPs) to enable sex-unbiased identification of microchimeric cells within tissues. In the second paper, Sing Sing Way’s group published an approach to deplete specific lineages of maternal cells in mice. While the researchers focused on immune cells, the approach could be used to study the functions of various microchimeric cell types.
It’s fascinating to think that I might harbor cells from a multitude of close relatives (even my grandmother or older siblings). The techniques presented in the two papers will help researchers better understand where these cells reside, where they come from, and what they are doing. I’m curious to see how this field develops and looking forward to reading Hidden Guests.
Single Nucleotide Polymorphism Typing Going Spatial: In Situ Padlock Probes Targeting mRNA Variants to Identify Haploidentical Cells within the Tissue Environment
In Clinical Chemistry, November 2025
From the group of Thomas Kroneis at the Medical University of Graz
Tolerance to non-inherited maternal antigen is sustained by LysM+ CD11c+ maternal microchimeric cells
In Immunity, September 2025
From the group of Sing Sing Way at Cincinnati Children’s Hospital Medical Center
Snippet by Katrina Woolcock
Image credit: Figure 1 from “Fetal microchimerism and maternal health: a review and evolutionary analysis of cooperation and conflict beyond the womb,” Boddy et al, BioEssays 2015 (CC BY-NC).