Differential Immune Response to Hydroxyapatite Precursors Under Inflammatory Pressure: In Vitro and In Vivo Studies

Area of Science:

• Biomaterials Science
• Immunology
• Regenerative Medicine

Background:

• Clinical success of calcium phosphate bone grafts (CPs) relies on immune response.
• Current biocompatibility tests on healthy subjects fail to predict performance in inflamed patients.
• Chronic inflammatory diseases affect bone graft efficacy.

Purpose of the Study:

• To investigate how inflammation affects the immune response to biomimetic CPs.
• To compare the biocompatibility of dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), and hydroxyapatite (HAp) under inflammatory conditions.
• To establish disease-relevant models for predicting bone graft clinical outcomes.

Main Methods:

• In vitro studies used human immune cells (monocytes, macrophages, lymphocytes) and mesenchymal stromal cells (MSCs), with and without pro-inflammatory activation.
• In vivo biocompatibility was assessed by subcutaneous implantation in rats, with or without Complete Freund's Adjuvant (CFA)-induced inflammation.
• Immune reactivity was evaluated by cytokine secretion (TNF-α, IL-1β) and histological analysis (capsule thickness, angiogenesis, necrosis).

Main Results:

• Under normal conditions, CPs showed minimal immune reactivity.
• Inflammation-activated macrophages induced acute reactions with increased TNF-α and IL-1β.
• In inflamed rats, DCPD promoted angiogenesis with minimal inflammation; HAp caused severe necrosis and rejection; OCP showed intermediate fibrosis and angiogenesis.
• All CPs developed thicker fibrous capsules in inflamed animals.

Conclusions:

• Inflammatory conditions critically impact CP biocompatibility.
• Materials deemed biocompatible in healthy models can induce fibrosis or rejection in inflamed patients.
• Disease-relevant, immune-challenged models are crucial for accurate prediction of bone graft clinical efficacy and safety.