This is an open-label, multicenter, prospective pilot study of CDX-301 with or without plerixafor as a stem cell mobilizer for allogeneic transplantation (stem cells that come from another person). HLA-matched sibling healthy volunteers (donors) and patients with protocol specified hematologic malignancies (recipients) will be enrolled.

Inclusion Criteria:

Donors:

• Read, understood and provided written informed consent and willing to comply with all study requirements and procedures
• 6 out of 6 HLA-matched sibling
• Negative test for human immunodeficiency virus (HIV), hepatitis B, and hepatitis C
• Both male and female patients of childbearing potential enrolled in this trial must use adequate birth control measures
• Subjects should be in generally good health and without significant medical conditions, based upon pre-study medical history, physical examination, electrocardiogram (ECG), chest X- ray, and laboratory tests
• Meets all criteria to serve as a mobilized blood cell donor in accordance with all applicable individual Transplant Center criteria

Recipient:

• Read, understood and provided written informed consent and willing to comply with all study requirements and procedures
• 6 out of 6 HLA-matched sibling
• Both male and female patients of childbearing potential enrolled in this trial must use adequate birth control measures

Diagnosis of one of following:

• Acute Myelogenous Leukemia (AML) in 1st remission or beyond
• Acute Lymphoblastic Leukemia (ALL) in 1st remission or beyond
• Chronic Myelogenous Leukemia (CML)
• Chronic Lymphoblastic Leukemia (CLL), relapsing after at least one prior regimen
• Myelodysplastic Syndrome (MDS), either intermediate 1,2, or high risk by IPI Scoring System or transfusion dependent
• Non-Hodgkins Lymphoma (NHL) or Hodgkins Disease (HD) in 2nd or greater complete remission, partial remission, or in relapse
• Meets all criteria to serve as a transplant recipient in accordance with all applicable individual Transplant Center criteria

Exclusion Criteria:

Donors:

• Unwilling or unable to give informed consent, or unable to comply with the protocol including required follow-up and testing
• Prior treatment with any rhuFlt3L product
• Any vaccination within 4 weeks prior to CDX-301 dosing
• Donation of blood within 8 weeks, or donation of plasma within 2 weeks prior to CDX-301 dosing
• Any experimental treatment within 4 weeks prior to CDX-301 dosing
• Use of systemic immunosuppressive agents (excluding topical steroids) within 12 months prior to CDX-301 dosing.
• History of first degree relatives with primary or secondary immunodeficiency to include type 1 diabetes, multiple sclerosis, rheumatoid arthritis, scleroderma or psoriasis
• History of tuberculosis infection
• Herpes zoster within 3 months prior to starting study drug
• Pregnant or nursing

Recipient:

• Unwilling or unable to give informed consent, or unable to comply with the protocol including required follow-up and testing
• Prior allogeneic transplant
• More than one prior autologous transplant
• Prior treatment with any rhuFlt3L product
• Any vaccination within 4 weeks prior to transplant
• Uncontrolled infection at the time of the transplant conditioning regimen
• Pregnant or nursing
• Any condition, which, in the opinion of the clinical investigator, would interfere with the evaluation of the study outcome