Linlin Yang ^1,2* Barbara Zerbato ³ Alex Pessina ³ Luca Brambilla ³ Virginia Andreani ¹ Stefanie Frey-Jakobs ¹ Manfred Fliegauf ¹ Mohamed-Ridha Barbouche ^4,5 Qiaoxia Zhang ² Ferdinando Chiaradonna ³ Michele Proietti ¹ Xin Du ² Bodo Grimbacher ¹

• ¹ Institute for Immunodeficiency, Center for Chronic Immunodeficiency, Freiburg University Medical Center, Freiburg, Germany
• ² Department of Hematology, Shenzhen Second People’s Hospital, The First Affiliated Hospital of Shenzhen University, Shenzhen, China
• ³ Department of Biotechnology and Biosciences, School of Sciences, University of Milano-Bicocca, Milano, Lombardy, Italy
• ⁴ Department of Microbiology, Immunology and Infectious Diseases, College of Medicine and Health Sciences, Arabian Gulf University, Manama, Bahrain
• ⁵ Laboratory of Transmission, Control and Immunobiology of Infection, Pasteur Institute of Tunis, University Tunis El Manar., Tunis, Tunisia

Background: Hypomorphic mutations in the gene Phosphoacetylglucosamine mutase 3 (PGM3) cause a glycosylation disorder leading to immunodeficiency. This disorder is often associated with recurrent infections and atopy. The exact etiology remains unclear. Objective: This study aims to characterize the phenotypes and immunological features associated with PGM3 insufficiency and to investigate potential disease mechanisms. Methods: A systematic review of 44 published cases with PGM3 variants was performed, followed by T-cell phenotyping of two patients with PGM3 variants. A genotype-phenotypic severity study was conducted by comparing the residual PGM3 expression of 12 reconstituted variants in human B cells. A PGM3 inhibitor was then used to assess its effect on CD4+ T cell proliferation and differentiation. Results: Patients identified with PGM3 variants frequently presented with recurrent infections and atopy, accompanied by reduced naïve CD4+ T cell counts. A genotypephenotype study showed that low levels of residual PGM3 expression correlates with disease severity. Notably, inhibition of PGM3 activity was observed to impair TCRmediated CD4+ T cell proliferation and the synthesis of UDP-GlcNAc, complex Nglycans, O-GlcNAc, glycolytic stress, and mitochondrial respiration during proliferation in a dose-dependent manner. Partial loss of PGM3 activity was observed to preferentially enhance Th1 and Th2 differentiation while attenuating Th17 and Treg differentiation, consistent with clinical observations.Yang | 5 Conclusion: PGM3 emerges as a critical regulator of CD4+ T cell proliferation and differentiation. These findings provide new insights into the diverse clinical manifestations and therapeutic development of PGM3 insufficiency.