Vertebrates protect themselves from pathogenic microorganisms using two systems called innate and acquired immunity (Strand, 2008). On the other hand, insects do not have acquired immunity and can only protect against foreign substances using innate immunity (Ferrandon et al., 2007). Innate immunity in insects can be divided into cellular and humoral defenses to prevent invasion by pathogens and parasites (Lemaitre and Hoffmann, 2007). Cellular defense responses include phagocytosis, in which granule cells engulf foreign substances if they are smaller than blood cells, and encapsulation if they are larger than blood cells where granule cells and plasma cells surround and sequester the foreign substances (Pech and Strand, 1996, Wiegand et al., 2000, Lavine and Strand, 2002). Additional cellular defenses can occur when a large number of foreign substances smaller than blood cells, such as bacteria, invade the insect body cavity resulting in nodule formation where granule cells that have phagocytosed the bacteria aggregate to form nodules (Stanley, 2006, Eleftherianos et al., 2009, Kim et al., 2009, Shrestha and Kim, 2009, Jiang et al., 2010).

Humoral defense reactions can be more complex involving infection control substances, lectins, and melanin pigmentation that envelop and isolate the surface of foreign substances with melanin pigments before encapsulation can take place (Nappi, 1973; Wago, 1982, 1983). Foreign substances phagocytosed by hemocytes are taken into phagocytic vacuoles where they are digested by enzymes (Kocks et al., 2003; Seong et al., 2006; Melcarne et al., 2019). Nappi et al. (1995) speculated that in Drosophila, encapsulated foreign bodies are not only isolated from the body, but also killed by reactive oxygen species released during subsequent melanization. Wago (1995) reported that in dipterous insects, the surface of the foreign body is wrapped with melanin pigment and isolated before the encapsulation occurs. Thus, it is believed that the humoral and cellular defense reactions do not necessarily function independently, but work in cooperation to efficiently eliminate foreign substances.

In the larva of the Mythimna separata (Ms), hyperspread cells (HSCs) specifically adhere to and extend into foreign substances, resulting in the formation of melanin (Kato et al., 2014). When Cotesia kariyai (Ck) 1st instar larvae are transplanted into Ms larva as foreign substances, HSCs adhere to the surface of the Ck larvae, and the surface of the foreign substances are melanized, followed by encapsulation by hemocytes.

The lepidopteran fat body synthesizes and releases several proteins, such as the pattern recognition protein hemolin and two immunolectin serine proteinases: prophenoloxidase activating proteinase and a serine proteinase inhibitor from the serpin family (Zhu et al., 2003).

The insect fat body has recently gained attention not only as a metabolic organ, but also as an immunologically active tissue. For example, in Ostrinia furnacalis, the C-type lectin OfIML-3, produced in the fat body, has been shown to be secreted into the hemolymph where it binds to hemocytes and foreign substances to promote encapsulation (Zhao et al., 2025). In Drosophila, the fat body-derived C-type lectins DL2 and DL3 are known to enhance the recognition and melanization of foreign substances (Ao et al., 2007). Furthermore, it has been suggested that digestive enzymes derived from the fat body may possess immune functions and contribute to the degradation of foreign substances (Pérez et al., 2021).

To better understand how melanized and encapsulated foreign substances are subsequently processed, we investigated the role of the insect fat body in the later stages of foreign substance elimination. By transplanting first instar Ck larvae into the body cavity of Ms larvae as foreign substances, then characterizing their interactions with the fat body following melanization and encapsulation, we observed a novel innate immune defense mechanism where fat body enzymes contribute to the degradation of foreign substances during direct contact.