Inclusion Criteria:

• Signed informed consent
• Ability and willingness to comply with the requirements of the study protocol
• Age ≥ 18 years
• Part 1 (dose escalation): Documented history of histologically confirmed CD20-positive, follicular NHL or marginal zone NHL
• Part 2 (expanded cohort): diagnosis of CD20-positive follicular NHL
• At least one previous treatment with a rituximab-containing regimen
• Measurable disease
• Either fresh or archived tumor specimen must be available for central confirmation of diagnosis and correlative studies
• ECOG performance status of 0 or 1
• Life expectancy of > 3 months
• For patients of reproductive potential, use of a reliable means of contraception (e.g., contraceptive pill, intrauterine device [IUD], barrier methods) throughout the trial and for 1 year following their final exposure to study treatment

Exclusion Criteria:

• Chemotherapy or radiotherapy within 28 days of Day 1
• Prior treatment with a monoclonal antibody directed against CD40
• Radioimmunotherapy or immunotherapy with a monoclonal antibody other than rituximab within 3 months of Day 1
• Prior treatment with an investigational drug within 28 days of Day 1
• Prior allogeneic bone marrow transplant
• Prior autologous hematopoietic stem cell transplant within 12 weeks of Day 1
• Concurrent systemic corticosteroid therapy
• Prior anaphylactoid or other serious reaction to rituximab that resulted in hospitalization or discontinuation of therapy, or both
• Evidence of clinically detectable ascites on Day 1
• Other invasive malignancies within 3 years prior to Day 1 except for adequately treated basal cell or squamous cell skin cancer, in situ carcinoma of the cervix, in situ breast cancer, in situ prostate cancer, or other cancer of which the patient has been disease-free for at least 3 years
• History or evidence on physical examination of CNS disease
• Active infection requiring parenteral antibiotics within 14 days of Day 1
• Major surgical procedure (excluding lymph node biopsy) or significant traumatic injury within 28 days prior to Day 1
• Pregnancy (positive pregnancy test) or lactation
• Serious, nonhealing wound, ulcer, or bone fracture
• Clinically significant cardiovascular disease, congestive heart failure, serious cardiac arrhythmia requiring medication within 1 year prior to Day 1. Grade II or greater peripheral vascular disease at study entry.
• Known human immunodeficiency virus (HIV) infection
• Known serious medical conditions, including cirrhosis, hepatitis C infection, and chronic obstructive or chronic restrictive pulmonary disease
• Any other severe, acute, or chronic medical or psychiatric condition, laboratory abnormality, or difficulty complying with protocol requirements that may increase the risk associated with study participation or study drug administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the patient inappropriate for this study