Maternal: 1. 18 years or older 2. Signed informed consent to participate 3. Diagnosis of moderate/severe SLE 4. Ongoing treatment with anifrolumab 5. Has reached or will reach steady state (~85 days postpartum, equivalent to at least 3 consecutive previous IV doses or at least 12 previous SC doses during the post-partum period) with anifrolumab by the time of study Day 1 (pre-dose milk collection) 6. Established lactation in the index post-partum period (breastfeeding or pumping for at least 4 weeks at time of Day 1 visit to ensure mature milk production) 7. Willing to breastfeed or pump regularly during the study period to maintain milk supply and exclusively pump breast milk for the 24-hour period of breast milk collection on Day 1 post IV dose 8. Plans to continue feeding infant breast milk at least throughout the duration of the study and is not weaning 9. Must be exclusively breast milk-feeding their infant (or if not exclusively breast milk-feeding, not providing more than 1 supplemental bottle of formula per day) at the time of enrollment and throughout the study period 10. Agrees to use only lanolin nipple cream during the sampling period Infant: 1. Gestational age at delivery ≥32 weeks 2. Birthweight > 10th percentile 3. Weight > 10th percentile at the time of enrollment

Maternal: 1. Received any investigational compound or approved biologic or biosimilar within 30 days or 5 half-lives (whichever is longer) prior to enrollment in the study 2. Diagnosis of lupus nephritis in the last 12 months 3. History of breast implants, breast augmentation, or breast reduction surgery that significantly impacts breastfeeding or collection of milk from 1 or both breasts 4. History of malignancy in the last 10 years 5. History of mastectomy 6. Evidence of mastitis or any other significant active infection at Day 1 (pre-dose) Infant: 1. Any abnormality noted or clinically significant medical condition, including cardiac, pulmonary, and liver disease, glucose instability, or active infection at the time of screening that, in the opinion of the investigator, may make implementation of the protocol or interpretation of the trial difficult or would put the infant participant at risk by participating in the study