Role of IgG4-mediated Suppression of Immune Effector Mechanisms in Human Filariasis

Abstract

Lymphatic filariasis (LF) is a major public health concern in tropical and subtropical countries. The infection affects more than 120 million people and has significant social and economic consequences on affected individuals and communities. LF is caused by the filarial parasites <em>Wuchereria bancrofti, Brugia malayi </em>and<em> Brugia timori</em> which are transmitted by mosquito vectors. Filarial parasites are known to be efficient modulators of their host’s immune system. To guarantee their own survival, they generate alongside the classical Th2 immune response, a strong regulatory phenotype with high levels of anti-inflammatory cytokines and elevated plasma levels of IgG4. This particular antibody was shown in different models to exhibit immunosuppressive properties and to be associated with the hyporesponsive states observed in LF infection. However, how IgG4 is involved in the pathogenesis of human filariasis is not well characterized. The present thesis aimed at analyzing the role of IgG4 antibody in the suppression of two immune effector mechanisms observed during LF infection: granulocyte activation and degranulation and complement activation. Moreover, the mechanisms sustaining IgG4-mediated immunosuppression were investigated. <br /> The first part of this thesis studied the impact of plasma and affinity-purified IgG/IgG4 fractions from endemic normals (EN), LF infected pathology patients (CP), asymptomatic microfilaraemic (Mf+) and amicrofilaraemic (Mf-) individuals on IgE/IL3/<em>BmAg</em> activated granulocytes and consequently degranulation. The activation and degranulation states were analyzed by monitoring the expression of CD63/HLADR and the release of granule contents (neutrophil elastase (NE), eosinophil cationic protein (ECP) and histamine) by flow cytometry and ELISA, respectively. The data demonstrated that granulocyte activation and degranulation were inhibited in the presence of plasma from EN and Mf+ individuals, whereas those of Mf- and CP presented no effect. This inhibitory capacity is associated with total IgG and non-IgG fractions of Mf+ patients but was abrogated when non-IgG factors were removed from EN plasma. Strikingly, the inhibitory effect in IgG positive fractions is related to IgG4 antibody. Furthermore, the results also revealed that, except in chronic pathology patients, IgG4 from EN, Mf+ and Mf- selectively reduced the activation of granulocyte neutrophils and basophils but not eosinophils. In the second part, this thesis addressed the question of the mechanisms by which IgG4 suppressed granulocyte functions. IgG4 from Mf+ patients, compared to those from EN and Mf-, demonstrated a high affinity to granulocytes, suggesting possible functional differences between IgG4 antibodies. Moreover, the suppression of granulocyte activation by IgG4 from Mf+ is mediated via FcgammaRI and FcgammaRII and after induction of the phosphorylation of the kinase SHIP1 but not Src and Syk. The third part of the thesis investigated the role of IgG4 antibodies in the modulation of complement activity during LF. The findings indicated that IgG1 and IgG2, present in plasma for Mf+ patients, displayed a reduced capacity to bind complement first component C1q compared to EN, Mf- and CP. Interestingly, the depletion of IgG4 from Mf+ plasma significantly increased the C1q binding capacity of IgG1 and IgG2 suggesting that IgG4 may function by preventing the binding of these pro-inflammatory antibodies to complement. <br /> Taken together these data provide evidences of the participation of IgG4 antibodies in the suppression of granulocyte and complement activities during lymphatic filariasis and also the importance of both qualitative and quantitative modulation of IgG4 in the pathophysiology of LF.