Identification of Novel Tumor-Associated Antigens in Chronic Lymphocytic Leukemia (CLL) by Serological Proteome Analysis (SERPA)

In this study, a serological proteome analysis (SERPA) was applied for the first time to identify novel tumor-associated antigens (Ags) capable of eliciting humoral immune responses in patients with chronic lymphocytic leukemia (CLL). SERPA has been demonstrated to be a valuable method to identify tumor associated Ags in several human solid and hematological malignancies. The identification and characterization of circulating antibodies (Abs) and corresponding Ags in CLL can provide useful information to understand cell transformation, predict clinical outcome, and develop immune-based interventions.

SERPA was performed in 21 untreated patients. Proteins extracted from purified CLL cells were separated by 2-D electrophoresis (2-DE) to obtain proteomic maps which were blotted with corresponding sera by Western Blot to reveal Ab-based reactivity with autologous proteins. To verify the CLL specificity of Abs recognition, 7 out of 21 maps were also probed with sera collected from 7 healthy donors (HD). For identification, Ag spots in WB were aligned with proteins in 2-DE maps. The protein spots corresponding to the assigned Ags were excised from the gel, trypsin digested and analyzed by peptide mass fingerprint by MALDITOF Mass Spectrometry (MS) with the software MASCOT. T cells from 6 CLL patients and 3 HD were stimulated with autologous ENOA-pulsed and control dendritic cells (DC) and evaluated by IFNγ ELISPOT assay. Ags surface expression was analyzed by flow cytometry. Statistical correlations were performed using t-test, Mann-Withney rank sum test and χ²-test.

Sixteen out of 21 CLL sera (76%) were immunoreactive and produced a total number of 45 Ag spots, whereas HD sera produced only 3 spots (p<.03). Eleven out of 16 (69%) reactive CLL sera recognized from 2 to 6 different Ags in each individual patients. MS analyses led to the identification of 16 different Ags and many of them were recognized by sera from different patients. Forty-eight percent of CLL sera reacted against α-Enolase (ENOA), whereas none of HD sera was ENOA reactive. The IGHV mutational status was available in 19 CLL patients: 10 were mutated (M), while 9 were unmutated (UM). Interestingly, ENOA was recognized by sera from 7/10 M patients (70%), but only by sera from 3/9 UM patients (33%). Cytofluoroimetric analyses performed in 7 patients showed that ENOA was undetectable on viable CLL cells surface, whereas it was translocated on the membrane of apoptotic CLL cells.

Statistical correlation analyses showed that immunoreactive CLL patients are characterized by an early stage of disease. Moreover, ENOA-reactive patients have a better preserved immune system because they have higher numbers of CD3⁺ (p=.02), CD3⁺/CD4⁺ (p=.03) and CD3⁺/CD8⁺ (p=.05) cells in the peripheral blood than ENOA-unreactive patients.

We also investigated the possibility to induce ENOA-specific T-cell immune responses in 6 CLL patients. ENOA-pulsed DC induced IFNγ production in 4/6 patients (66%). The response was ENOA and CLL specific because: 1) it was not induced by unpulsed DC or DC pulsed with an irrelevant protein; 2) it was not induced when T cells from 3 HD were stimulated with autologous ENOA-pulsed DC. Interestingly, ENOA Abs were detectable by SERPA in 3 out of 4 (75%) patients with ENOA-induced T-cell responses, whereas they were undetectable in patients with unresponsive T cells. Correlations with the IGHV mutational status showed that all patients with ENOA-reactive T cells were M.

These results indicate that ENOA is able to elicit specific humoral and cellular immune responses suggesting that this protein can be a promising biomarker and a potential target for immunotherapy in CLL.

Massaia:Novartis Farma S.p.A: Honoraria, Research Funding.