High affinity cross-reacting mAb generated by minimal mimicry: implications for the pathogenesis of anti-nuclear autoantibodies and immunosuppression

High-valency Anti-CD99 Antibodies Toward the Treatment of T Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive form of leukemia that currently requires intensive chemotherapy. While childhood T-ALL is associated with high cure rates, adult T-ALL is not, and both are associated with significant short- and long-term morbidities. Thus, less toxic and effective strategies to treat T-ALL are needed. CD99 is overexpressed on T-ALL blasts at diagnosis and at relapse. Although targeting CD99 with cytotoxic antibodies has been proposed, the molecular features required for their activity are undefined. We identified human antibodies that selectively bound to the extracellular domain of human CD99, and the most potent clone, 10A1, shared an epitope with a previously described cytotoxic IgM antibody. We engineered clone 10A1 in bivalent, trivalent, tetravalent, and dodecavalent formats. Increasing the antibody valency beyond two had no effects on binding to T-ALL cells. In contrast, a valency of ≥3 was required for cytotoxicity, suggesting a mechanism of action in which an antibody clusters ≥3 CD99 molecules to induce cytotoxicity. We developed a human IgG-based tetravalent version of 10A1 that exhibited cytotoxic activity to T-ALL cells but not to healthy peripheral blood cells. The crystal structure of the 10A1 Fab in complex with a CD99 fragment revealed that the antibody primarily recognizes a proline-rich motif (PRM) of CD99 in a manner reminiscent of SH3-PRM interactions. This work further validates CD99 as a promising therapeutic target in T-ALL and defines a pathway toward the development of a selective therapy against T-ALL.

Change in the accessibility of an epitope of the human granulocyte-colony stimulating factor after binding to receptors

In a previous work we demonstrated that monoclonal antibody (mAb) 8C2 recognized a human granulocyte-colony stimulating factor (hG-CSF) region left unmasked after binding to placenta receptors, whereas mAb 6E3 defined a receptor-buried epitope. Herein we examined the role of these antigenic regions on the proliferative response induced by hG-CSF on a myeloid leukaemia cell line. Both mAbs significantly inhibited the hG-CSF-induced cell growing, although epitope 8C2 but not 6E3 remained exposed in hG-CSF:cell receptor complexes. When cytokine:receptor complexes already formed at 4 degrees C were incubated 1 h at 37 degrees C under conditions preventing the internalization, a significant reduction in the amount of accessible 8C2 epitopes was evident. However, this effect was not observed when mAb 8C2:hG-CSF complexes previously bound to cells were incubated at 37 degrees C. Thus, results suggest that a receptor oligomerization process could account for the temperature-induced epitope 8C2 masking. The identification of epitope 8C2 accomplished by synthesis of overlapping octapeptides, revealed that it is formed by sequences 39-52 and 155-164, both in close proximity in the three-dimensional structure of the hG-CSF molecule. Since part of this region has been proposed as a second binding site to receptors, we infer that the change of epitope 8C2 accessibility could be the result of either receptor aggregation or epitope binding to another receptor. In addition, our data support the hypothesis that a ligand-induced receptor oligomerization is required for transduction of cytokine signals.

Survivin exists in immunochemically distinct subcellular pools and is involved in spindle microtubule function

Survivin is a member of the inhibitor of apoptosis gene family that has been implicated in both apoptosis inhibition and regulation of mitosis. However, the subcellular distribution of survivin has been controversial and variously described as a microtubule-associated protein or chromosomal passenger protein. Here, we show that antibodies directed to the survivin sequence Ala3-Ile19 exclusively recognized a nuclear pool of survivin that segregated with nucleoplasmic proteins, but not with outer nuclear matrix or nuclear matrix proteins. By immunofluorescence,nuclear survivin localized to kinetochores of metaphase chromosomes, and to the central spindle midzone at anaphase. However, antibodies to Cys57-Trp67 identified a cytosolic pool of survivin,which associated with interphase microtubules, centrosomes, spindle poles and mitotic spindle microtubules at metaphase and anaphase. Polyclonal antibodies recognizing survivin epitopes Ala3-Ile19,Met38-Thr48, Pro47-Phe58 and Cys57-Trp67 identified both survivin pools within the same mitotic cell. A ratio of ∼1:6 for nuclear versus cytosolic survivin was obtained by quantitative subcellular fractionation. In synchronized cultures, cytosolic survivin abruptly increased at mitosis, physically associated with p34cdc2, and was phosphorylated by p34cdc2 on Thr34, in vivo. By contrast, nuclear survivin began to accumulate in S phase, was not complexed with p34cdc2 and was not phosphorylated on Thr34. Intracellular loading of a polyclonal antibody to survivin caused microtubule defects and resulted in formation of multipolar mitotic spindles, but did not interfere with cytokinesis. These data demonstrate that although both reported localizations of survivin exist in mitotic cells, the preponderant survivin pool is associated with microtubules and participates in the assembly of a bipolar mitotic spindle.