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.