Introduction Paternal lymphocyte immunization (PLI) is a cell-based immunotherapy that stimulates activation and response of maternal immune system. 1 PLI is performed using mononuclear cells separated from peripheral blood of husband in a Ficoll solution. 2,3 These cells are then subcutaneously injected to the wife periodically in an effort to prepare maternal immune system so that pregnancy could take place. PLI was initially carried out in 1981 in several countries to resolve clinical cases, such as recurrent miscarriage and failure of in-vitro fertilization (IVF), with significant progress in pregnancy and live birth rates. 1 In patients with recurrent miscarriage treated with LIT, the presence of ANA and TgAb was a risk factor for further pregnancy loss. Perinatal outcomes in those whose pregnancies continued were favourable. 4 Four published meta-analysis found a beneficial effect of immunotherapy with lymphocytes in cases of RM, with significant improvements in the rate of live births. 5 PLI can be used for immunotherapy in patients with recurrent spontaneous abortion. PLI has good efficacy and safety 6 and can be used to treat recurrent spontaneous abortions (RSA) effectively. Furthermore, the detection of blocking antibodies showed a positive prediction on pregnancy outcome. 7 A meta-analysis of recurrent miscarriage cases in a study involving 1,738 patients (a PLI group of 739 patients and a control group of 999 patients) revealed that PLI significantly improved live birth rate in recurrent miscarriage cases (OR 4.67, 95% CI 3,70-5,90). 8 The efficacy of PLI for women with idiopathic infertility is around 43-73%. Another study has shown an efficacy of PLI of 38% in recurrent spontaneous miscarriages and 12% in idiopathic infertility. 9 A study of 749 patients with recurrent miscarriages seeking treatment from October 2009 to June 2013 in China reported a successful and statistically significant conception rate of 89.7% in women treated with PLI. 7 The mechanism underlying the success of PLI is not clear. The aim of the present study was to analyse the effect of PLI on antisperm antibody (ASA) titre, regulatory T-cell population and serum IL-10 levels in women with unexplained infertility. Subjects and Methods This research method was observational, with a pre-post design. The study group consisted of 12 married women with ASA titres >1:128 who came to Sayyidah Mother and J Pak Med Assoc (Suppl. 2) 11th Malaysia-Indonesia-Brunei Medical Science Conference 2019 S-95 1 PhD Student, BioMedical Sciences, 2 Department of Biology, 4 Department of Molecular Biology and BioChemistry, Universitas Indonesia, Jakarta, 3 Budhi Jaya Mother and Child Hospital, Jakarta, Indonesia. Correspondence: Mohamad Sadikin. Email: mosadik48@yahoo.fr RESEARCH ARTICLE Effects of paternal lymphocyte immunization in women with unexplained infertility Nani Sari Murni, 1 Indra Gusti Mansur, 2 Ichramsyah A Rachman, 3 Mohamad Sadikin 4 Abstract Objective: Infertility in women could be a result of an excessive production of antisperm antibody (ASA). Paternal lymphocyte immunization (PLI) could decrease the ASA levels, but the mechanism is still unclear. The aim of this study was to analyse the impact of a PLI-induced ASA decline on regulatory T-cell populations and serum interleukin-10 (IL-10) levels in women with unexplained infertility. Methods: Samples were obtained from patients who came to Sayyidah Mother and Child Hospital in Jakarta from July 2018 to April 2019 with infertility problems. The inclusion criterion for this study was unexplained infertility. Each patient was examined for ASA titres using husband's sperm auto-agglutination test (HSAaT) method, and patients with ASA titres >1:128 were given PLI subcutaneously every 3 weeks. ASA titres were evaluated again 2 weeks after PLI with HSAaT. A total of 12 samples were analysed. Regulatory T-cell populations were evaluated using flow cytometry and human forkhead box P3 FoxP3 staining kit of Biotech and Device, and serum IL-10 was determined using an Abcam ELISA kit. The data were analysed using Wilcoxon and Spearman tests. Results: PLI decreased serum ASA and percentage of regulatory T cells (p = 0.023). The decrease in ASA and subsequent decrease in regulatory T cell population was due to the strong negative correlation between regulatory T cells and IL-10 (r = -0.817, p = 0.004). Conclusion: The decline in ASA was associated with a decrease in regulatory T cells due to a negative correlation with IL-10levels. Keywords: Regulatory T cell, IL-10, sperm agglutination ASA, Infertility. (JPMA 71: S-95 [Suppl. 2]; 2021)