A Study of ALPN-303 in Adult Healthy Volunteers

Summary of Inclusion Criteria: * Able and willing to provide written informed consent * Body mass index 18 to 32 kg/m2 * Agree to avoid strenuous physical activity for 2 days prior to each study visit * Agree to use highly effective contraception during the study (all participants) and for 90 days after study drug dosing (males) or for 60 days after study drug dosing (females) * Male participants must refrain from donating sperm during the study and for 90 days after study drug dosing * Female participants must not be pregnant or breastfeeding and must refrain from donating ova during the study and for 60 days after study drug dosing * Must have completed vaccination against SARS-CoV-2 at least 4 weeks prior to study drug dosing Summary of Exclusion Criteria: * Any current disease, condition, or treatment that could interfere with the study, interfere with interpretation of the data, or pose an unacceptable risk to the participant * History or symptoms of significant psychiatric disease; mild, resolved depression or anxiety is acceptable * History of immunological disorders, auto-immune disorders, acquired or congenital immune deficiency; see protocol for clarifications/exceptions * History of significant hepatic or renal disease or impairment * Evidence of an active or suspected cancer or a history of malignancy within the previous 3 years; see protocol for exceptions * History of major organ transplantation with an existing functional graft * Use or receipt of prescription medications, over-the-counter medications, herbal remedies, or investigation products (study drugs) within protocol-defined timeframes prior to study entry; hormone replacement therapy initiated at least 2 months prior to Screening is acceptable * Significant loss of blood or blood product (including donation) over 500 mL or transfusion of any blood product during Screening or within 3 months of Screening * Unwilling to refrain from alcohol use ≥ 48 hours prior to Study Day -1. * Known hypersensitivity, allergy, or intolerance to ALPN-303, Fc-based biologic therapy, or any of the excipients contained in the ALPN-303 formulation * Immunization with any live vaccine within 6 weeks prior to study drug administration, or expected to require any live vaccines during the study or within 6 weeks after receiving study drug * Acceptable laboratory assessments at Screening and Day -1 * Positive screen for drugs of abuse or alcohol at Screening or Day -1 * Positive tests for infectious disease (HIV, hepatitis B, hepatitis C, SARS-CoV-2) * Acute infection during or within 4 weeks prior to Screening * History of frequent or atypical infections as defined per protocol; signs or symptoms of immunodeficiency * History or presence of any chronic infectious condition * Cardiac risk factors, as defined per protocol