This study aims to develop and refine a microRNA (miR) biomarker panel that can be used to phenotype net immune state after heart transplantation using circulating miRs (associated with drug doses and levels). These miRs will be used to characterize the overall immune state in adult heart transplant patients and predict patients that will go on to develop infection and rejection. MicroRNAs (miRs) are small, non-coding RNA molecules that regulate gene expression and serve as molecular biomarkers found in the circulation.

The study objectives will be accomplished in a prospective, multicenter observational, longitudinal cohort study that includes ~250 Heart Transplant patients from the United States. Patients will be screened for eligibility and enrolled ~1 month (± 2 weeks) after transplant. Study participation will last 36 months.

All patients will follow the center's standard of care surveillance schedule after transplant. Blood samples will be collected for miR evaluation at:

1. specified time intervals after transplant and
2. when a clinical event of interest occurs, including treated rejection, or infection.

Research samples will be collected and used to evaluate miR expression as well as other biomarkers related to heart transplantation and immunosuppression medications. Additional data collection will include demographics, medical history, medications, human leukocyte (HLA)/donor specific antibody (DSA) evaluations, endomyocardial biopsy (EMB) echocardiography, donor-derived cell-free DNA (dd-cfDNA), and other post-transplant events and testing.

This work will form the basis for a non-invasive, genomic blood test that can be used to monitor patients after heart transplant to mitigate complications of over-immunosuppression, such as infection, without increasing the risks of under-immunosuppression, such as rejection.

Inclusion Criteria:

• Age ≥ 18 years at enrollment
• Receipt of orthotopic heart transplant (OHT) within the prior 1 month ± 2 weeks
• Planned follow-up at the transplant center for a minimum of one-year.
• Patient able and willing to comply with the study visit schedule, study procedures, and study requirements.

Exclusion Criteria:

• Recipient of a multi-organ transplant
• History of prior solid organ transplant before the index heart transplant
• Ongoing mechanical circulatory support or hemodynamic instability (e.g., inotrope or vasopressor therapy)
• Ongoing need for renal replacement therapy and/or dialysis
• Active infection requiring either a) hospitalization b) treatment with antimicrobial therapy or c) reduction in immunosuppression
• Active rejection being treated with intravenous medications or plasmapheresis

Patients ≥ 18 years of age from geographically and socioeconomically diverse regions of the U.S who have undergone orthotopic heart transplant (OHT)

Patients will be screened for eligibility and enrolled within ~ 1 month after heart transplant