A Study of Intravenous CPTX2309 in Healthy Participants and Participants With Moderate to Severe Rheumatoid Arthritis (RA) or Systemic Lupus Erythematosus (SLE)

Inclusion Criteria: * Male and female participants who are healthy as determined by the investigator based on review of medical history, physical examination, and clinical laboratory tests obtained during the screening period. * Participant is willing and able to adhere to the study visit schedule and other protocol requirements. RA Only: * Clinical diagnosis of RA and fulfilling the 2010 ACR/EULAR classification criteria for RA * Presence of rheumatoid factor or ACPA above the ULN * Confirmation of at least moderate active disease at screening with the presence of at least 6 swollen and 6 tender joints at screening using the 68 (tender)/66 (swollen) joint count OR the presence of at least 3 joints with active synovitis via MRI assessment. SLE Only: * Clinical diagnosis of SLE and fulfilling the 2019 EULAR/ACR classification criteria for SLE * Positive ANA\>=1:80 and the presence of at least one of the following autoantibodies above the upper limit of normal (ULN): anti-double standard DNA (dsDNA) or anti-Smith (Sm). Exclusion Criteria: * Evidence of organ dysfunction or any clinically significant deviation from normal in physical examination, vital signs, or clinical laboratory tests beyond what is consistent with a healthy population in the region in which the study is conducted. * Use of any investigational medical device or investigational drug within 30 days or 5 half-lives of the investigational drug (whichever is longer) prior to the first administration of CPTX2309. RA Only: \- Participants diagnosed with Felty's syndrome SLE Only: * Active neuropsychiatric SLE, as defined by the CNS portion of SLEDAI at Screening, or signs of symptoms of neuropsychiatric SLE within 6 months prior to Screening (lupus headache permissible) * Note: Other protocol-defined inclusion/exclusion criteria apply