Adult Healthy Volunteers Study to Evaluate ALPN-101 Safety

Inclusion Criteria: 1. Able to participate and willing to give written informed consent. 2. Healthy adult participant 18 to 65 years of age, inclusive, at the time of informed consent. 3. BMI between 18 and 30 kg/m2, inclusive. 4. Male participants must agree to use a highly effective method of contraception consistently and correctly during the treatment period and for at least 30 days after the last dose of study intervention and to refrain from donating sperm during this period. 5. Female participants must not be pregnant or breastfeeding Exclusion Criteria: General Health Exclusions 6. Any concomitant disease, condition or treatment that could interfere with the conduct of the study or that would, in the opinion of the Investigator or Sponsor, pose an unacceptable risk to the participant in the study or interfere with the interpretation of study data. 7. History or symptoms of significant psychiatric disease in the opinion of the Principal Investigator, including but not limited to depression and schizophrenia. 8. History of immunological disorders, auto-immune disorders, acquired or congenital immune deficiency. 9. History of significant hepatic or renal disease or impairment. 10. Evidence of an active or suspected cancer or a history of malignancy within the previous 3 years, except for the following, which did not require systemic therapy and are considered cured: nonmelanoma skin cancer, curatively treated localised prostate cancer or other in situ cancer. 11. History of major organ transplantation with an existing functional graft. Therapeutic and Intervention-Related Exclusions 12. Use of prescription medications or herbal remedies within 14 days or 5 elimination half-lives (whichever is longer) of study intervention administration or use of over-the-counter medications (OTC) within 7 days of study intervention administration (apart from recommended doses of vitamin/mineral supplements, OTC analgesics, etc., approved by the Investigator and Sponsor). Participants who have been on hormone replacement therapy (HOURST) for a period of at least 2 months will not be excluded from the study, provided the HOURST regimen remains unchanged during the conduct of the study. 13. Any concomitant medications that prolong QT/QTc interval. 14. Dosed with an investigational drug within 30 days or 5 elimination half-lives (whichever is longer) prior to study drug administration. 15. Received any systemic steroid within 1 month or systemic immunosuppressant agent within 6 months prior to study drug administration. 16. Received any antibody therapy or biologic products within 6 months prior to study drug administration. 17. Significant loss of blood including blood donation over 500 mL or transfusion of any blood product within 3 months before the Screening Visit. 18. Unwilling to refrain alcohol use 48 hours prior to your admission into the clinical research unit. 19. Known hypersensitivity, allergy or intolerance to the study drug or any of the excipient contained in the intervention formulation. 20. Immunisation with any vaccine within 6 weeks prior to study drug administration. 21. For participants enrolled in cohorts in which KLH will be administered: known administration of KLH within 6 months prior to study drug administration; known seafood or shellfish allergy, or known hypersensitivity or allergy to KLH Laboratory Exclusions 22. Any clinically significant (at the discretion of the Investigator) abnormalities in laboratory test results, including complete blood count, chemistry panels and urinalysis. If out of range at the Screening Visit and/or admission to the CRU and deemed clinically significant by investigator, the tests may be repeated once on a separate day. 23. Estimated creatinine CL \< 90 mL/minute at the Screening Visit 24. Positive test at the Screening Visit and/or at baseline for drugs of abuse (e.g., phencyclidine, opiates, benzodiazepines, barbiturates, amphetamines, methamphetamines, cocaine and THC). The repeat test is permitted once for a suspected false positive test. 25. Positive alcohol breath test at the Screening Visit and/or at baseline. 26. Positive result on tests for HIV-1 and/or HIV-2 antibody, anti-hepatitis B virus antibodies (core antibody \[HBcAb\], surface antigen \[HBsAg\]) or anti-hepatitis C virus antibody. 27. Positive serum pregnancy test at the Screening Visit or admission to the CRU. Infection-related Exclusions. 28. Acute infection including viral infections in the preceding 6 weeks 29. History or presence of any chronic infectious condition, including but not limited to tuberculosis or parasitic infections. Evidence of tuberculosis may include either a positive tuberculosis blood test (by a positive QuantiFERON® TB-Gold test or a positive tuberculin skin test) or chest X-ray radiographic findings consistent with active or latent tuberculosis. Cardiovascular Exclusions 30. Supine sustained (i.e. 3 independent measurements within 30 min) systolic blood pressure greater than 140 or less than 90 mmHg. 31. Supine sustained (i.e. 3 independent measurements within 30 min) diastolic blood pressure greater than 90 or less than 40 mmHg. 32. Sustained (i.e. 3 independent measurements within 30 min) heart rate (pulse) greater than 100 or less than 40 beats per minute (bpm). 33. QTcF \< 300 msec or ≥ 450 msec. 34. Personal and/or family history of congenital long QT syndrome or sudden cardiac death