A Study of BI-1206 in Combination With Rituximab With or Without Acalabrutinib in Subjects With Indolent B-Cell NHL

Inclusion Criteria: 1. Are ≥ 18 years of age by initiation of study treatment. 2. Have B-cell NHL proven by histology, with histological subtypes limited to follicular lymphoma (FL) (except FL grade 3B), MCL and marginal zone lymphoma (MZL) 3. Have measurable nodal disease 4. Are willing to undergo lymph node biopsies or biopsies of other involved tissue 5. Have relapsed disease or disease refractory to conventional treatment or for which no standard therapy exists 6. Have received at least one line of conventional previous therapy which must include at least one rituximab-based regimen 7. Have a life expectancy of at least 12 weeks 8. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 9. Have CD20+ malignancy 10. Have hematological and biochemical indices within prespecified ranges Exclusion Criteria: 1. Have had an allogenic bone marrow or stem cell transplant within 12 months 2. Have presence of active chronic graft versus host disease 3. Have current leptomeningeal lymphoma or compromise of the central nervous system 4. Have transformed lymphoma from a pre-existing indolent lymphoma 5. Have Waldenstrom's Macroglobulinemia or FL grade 3B, 6. Need systemic doses of prednisolone \>10 mg daily (or equipotent doses of other corticosteroids) while on the study trial other than as pre-medication. 7. Have known or suspected hypersensitivity to rituximab or BI-1206 8. Have cardiac or renal amyloid light-chain amyloidosis 9. Have received any of the following: 1. Chemotherapy or small molecule products with 2 weeks of first dose of BI-1206 2. Radiotherapy (except for focal symptomatic control of lymphadenopathy) within 4 weeks 3. Immunotherapy within 8 weeks 4. Previous lines of treatment containing BTK inhibitors for Subjects receiving BI-1206 in combination with rituximab and acalabrutinib 10. Have ongoing toxic manifestations of previous treatments. 11. Have the ability to become pregnant (or already pregnant or lactating/breastfeeding). 12. Have had major surgery from which the subject has not yet recovered. 13. Are at high medical risk because of non-malignant systemic disease including active infection on treatment with antibiotics, antifungals or antivirals. 14. Are serologically positive for hepatitis B, hepatitis C or human immunodeficiency virus (HIV). 15. Have an active, known or suspected autoimmune disease. 16. Have concurrent congestive heart failure, prior history of class III/ IV cardiac disease (New York Heart Association \[NYHA\]) 17. Have current malignancies of other types