Inclusion Criteria:

Subjects must meet all of the following criteria to be eligible to participate in the
study:

- Confirmed diagnosis of HCL

- Measurable disease

- Patient's must have had at least 2 prior systemic therapies. There must have been
at least 2 prior courses of purine analog, or 1 if the response to this course lasted
<2 years, or if the patient had unacceptable toxicity to purine analog.

- ECOG performance status of 0-2.

- Patients with other cancers who meet eligibility criteria and have less than 5 years
of disease free survival will be considered on a case-by-case basis

- Life expectancy of greater than 6 months, as assessed by the principal investigator

- Must be able to understand and sign informed consent

- Must be at least 18 years old

- Female and male patients must agree to use an approved method of contraception during
the study

- Stage of Disease:

At least one of the following indications for treatment:

- Neutropenia (ANC <1000 cells/μL),

- Anemia (Hgb <10g/dL),

- Thrombocytopenia (Plt <100,000/μL),

- An absolute lymphocyte count of >20,000 cells/μL, or

- Symptomatic splenomegaly

Exclusion Criteria:

Subjects meeting any of the following criteria are not eligible for participation in the
study:

- History of allogeneic bone marrow transplant

- Documented and ongoing central nervous system involvement with their malignant
disease (history of CNS involvement is not an exclusion criterion)

- Pregnant or breast-feeding females

- Patients whose plasma contains either a significant level of antibody to CAT-8015 as
measured by ELISA, or antibody that neutralizes the binding of CAT-8015 to CD22 as
measured by a competition ELISA.

- HIV positive serology (due to increased risk of severe infection and unknown
interaction of CAT-8015 with antiretroviral drugs

- Hepatitis B surface antigen positive

- Uncontrolled, symptomatic, intercurrent illness including but not limited to:
infections requiring systemic antibiotics, congestive heart failure, unstable angina
pectoris, cardiac arrhythmia, psychiatric illness, or social situations that would
limit compliance with study requirements.

Hepatic function Serum transaminases (either ALT or AST) or bilirubin:

- ≥ Grade 2, unless bilirubin is due to Gilbert's disease

Renal function:

- Serum creatinine clearance ≤ 60mL/min as estimated by Cockroft-Gault formula

Hematologic function:

- The ANC < 1000/cmm, or platelet count <50,000/cmm, if these cytopenias are not judged
by the investigator to be due to underlying disease (i.e. potentially reversible with
anti-neoplastic therapy)

- A patient will not be excluded because of pancytopenia ≥ Grade 3, or erythropoietin
dependence, if it is due to disease, based on the results of bone marrow studies

- Baseline coagulopathy is greater than or equal to Grade 3 unless due to anticoagulant
therapy

Pulmonary function:

- Patients with < 50% of predicted forced expiratory volume (FEV1) or <50% of predicted
diffusing capacity for carbon monoxide (DLCO), corrected for hemoglobin concentration
and alveolar volume. Note: Patients with no prior history of pulmonary illness are
not required to have PFTs. FEV1 will be assessed after bronchodilator therapy.

Recent prior therapy:

- Cytotoxic chemotherapy, corticosteroids (except stable doses of prednisone), whole
body electron beam radiation therapy, hormonal, biologic or other systemic therapy,
or any investigational therapy of the malignancy for 3 weeks prior to entry into the
trial

- Less than or equal to 1 month prior monoclonal antibody therapy (i.e. rituximab)

- Patients who are receiving or have received radiation therapy less than 3 weeks prior
to study entry will be not be excluded providing the volume of bone marrow treated is
less than 10% and also the patient has measurable disease outside the radiation port

- Any history of prior pseudomonas-exotoxin(PE)immunotoxin administration