Inclusion Criteria:

          -  signed written informed consent obtained prior to study procedures

          -  histologically-confirmed relapsed or refractory CD20+ B-cell lymphoma of one of the
             following types, with documented disease progression or recurrence following the
             immediate prior therapy: DLBCL (regardless of cell of origin or underlying molecular
             genetics), MCL, PMBCL, Gr3b-FL, TH-FL (prior dx of FL before transforming to DLBCL).

          -  biopsy-confirmed CD20+ expression of the underlying malignancy with disease
             progression following immediate prior therapy

          -  at least 1 measurable lesion on imaging.

          -  must have received adequate prior therapy for the underlying CD20+ B-cell lymphoma,
             defined as an anti-CD20 mAb in combination with an anthracycline-containing
             chemotherapy regimen (i.e. chemo-immunotherapy) and at least one of the following:

               -  biopsy-proven refractory disease after frontline chemo-immunotherapy

               -  relapse within 1 year from frontline chemo-immunotherapy and ineligible for
                  autologous hematopoietic stem cell transplant (auto-HSCT)

               -  for subjects with DLBCL, PMBCL, and Gr3b-FL: relapsed or refractory disease
                  following at least 2 prior regimens or following an auto-HSCT

               -  for subjects with TH-FL: relapsed or refractory disease following at least 2
                  prior regimens or following an auto-HSCT. At least 1 prior regimen with an
                  anti-CD20 mAb in combination with chemotherapy is required following documented
                  transformation

               -  for subjects with MCL (confirmed with cyclin D1 expression or evidence of
                  t(11;14) by cytogenetics, fluorescent in situ hybridization (FISH) or polymerase
                  chain reaction (PCR): relapsed or refractory disease after at least 1 prior
                  regimen with chemo-immunotherapy (prior auto-HSCT is allowable)

          -  ECOG 0 or 1

          -  life expectancy of at least 6 months

          -  platelet count greater than 50,000/µL

        Exclusion Criteria:

          -  known active central nervous system (CNS) involvement by malignancy.

          -  prior treatment as follows:

               -  alemtuzumab within 6 months of enrollment

               -  fludarabine, cladribine, or clofarabine within 3 months of enrollment

               -  external beam radiation within 2 weeks of enrollment

               -  mAb (including rituximab) within 2 weeks of enrollment

               -  other lymphotoxic chemotherapy (including steroids except as below) within 2
                  weeks of enrollment

               -  experimental agents within 3 half-lives prior to enrollment, unless progression
                  is documented on therapy

          -  clinically significant cardiac disease

          -  clinically significant active infection

          -  clinically significant CNS disorder

          -  clinical history, prior diagnosis, or overt evidence of autoimmune disease

          -  known bone marrow involvement due to underlying malignant disease, in dose-escalation
             phase only