Osteopontin (OPN), a matricellular immunomodulatory cytokine highly expressed by myelomonocytic cells, is known to regulate immune cell migration, communication, and response to brain injury. Enhanced cerebral recruitment of monocytes achieved through glatiramer acetate (GA) immunization or peripheral blood enrichment with bone marrow (BM)-derived CD115⁺ monocytes (Mo^BM) curbs amyloid β-protein (Aβ) neuropathology and preserves cognitive function in murine models of Alzheimer's disease (ADtg mice). To elucidate the beneficial mechanisms of these immunomodulatory approaches in AD, we focused on the potential role of OPN in macrophage-mediated Aβ clearance. Here, we found extensive OPN upregulation along with reduction of vascular and parenchymal Aβ burden in cortices and hippocampi of GA-immunized ADtg mice. Treatment combining GA with blood-grafted Mo^BM further increased OPN levels surrounding residual Aβ plaques. In brains from AD patients and ADtg mice, OPN was also elevated and predominantly expressed by infiltrating GFP⁺- or Iba1⁺-CD45^high monocyte-derived macrophages engulfing Aβ plaques. Following GA immunization, we detected a significant increase in a subpopulation of inflammatory blood monocytes (CD115⁺CD11b⁺Ly6C^high) expressing OPN, and subsequently, an elevated population of OPN-expressing CD11b⁺Ly6C⁺CD45^high monocyte/macrophages in the brains of these ADtg mice. Correlogram analyses indicate a strong linear correlation between cerebral OPN levels and macrophage infiltration, as well as a tight inverse relation between OPN and Aβ-plaque burden. In vitro studies corroborate in vivo findings by showing that GA directly upregulates OPN expression in BM-derived macrophages (MФ^BM). Further, OPN promotes a phenotypic shift that is highly phagocytic (increased uptake of Aβ fibrils and surface scavenger receptors) and anti-inflammatory (altered cell morphology, reduced iNOS, and elevated IL-10 and Aβ-degrading enzyme MMP-9). Inhibition of OPN expression in MФ^BM, either by siRNA, knockout (KO^OPN), or minocycline, impairs uptake of Aβ fibrils and hinders GA's neuroprotective effects on macrophage immunological profile. Addition of human recombinant OPN reverses the impaired Aβ phagocytosis in KO^OPN-MФ^BM. This study demonstrates that OPN has an essential role in modulating macrophage immunological profile and their ability to resist pathogenic forms of Aβ.

T helper 9 (T_H9) cells are important for the development of inflammatory and allergic diseases. The T_H9 transcriptional network converges signals from cytokines and antigen presentation but is incompletely understood. Here, we identified TL1A, a member of the TNF superfamily, as a strong inducer of mouse and human T_H9 differentiation. Mechanistically, TL1A induced the expression of the transcription factors BATF and BATF3 and facilitated their binding to the Il9 promoter leading to enhanced secretion of IL-9. BATF- and BATF3-deficiencies impaired IL-9 secretion under T_H9 and T_H9-TL1A-polarizing conditions. In vivo, using a T-cell transfer model, we demonstrated that TL1A promoted IL-9-dependent, T_H9 cell-induced intestinal and lung inflammation. Neutralizing IL-9 antibodies attenuated TL1A-driven mucosal inflammation. Batf3^-/- T_H9-TL1A cells induced reduced inflammation and cytokine expression in vivo compared to WT cells. Our results demonstrate that TL1A promotes T_H9 cell differentiation and function and define a role for BATF3 in T-cell-driven mucosal inflammation.