Nivolumab Plus Rituximab in First-line Follicular Lymphoma gr 1-3A

This study will involve participants with a condition called Follicular NonHodgkin Lymphoma (Follicular Lymphoma). The main purpose of this study is to see if it is safe to give drug Nivolumab before and in combination with drug Rituximab and to see how effective Nivolumab is in patients who have had no previous drug treatment for their lymphoma. In particular, we will be monitoring for any specific side effects which may be increased by adding Nivolumab to Rituximab treatment, including monitoring of the immune system. Participants will be reviewed at baseline and prior to each cycle of treatment for toxicity, scans will be performed at baseline, after 4 cycles of nivolumab, after 8 cycles of nivolumab +/rituximab and at 6 months post induction treatment phase and following completion of treatment, participants will be followed up for a total of 5 years (every 3 months for 2 years, every 6 months for 3 years). In participants with relapsed disease, these will be followed for survival every 3 months.