Inflammation Driven Cancer / Tumor Associated Macrophages / Tumor Microenvironment · Journal article

Apoptosis inhibitor of macrophage (AIM/CD5L) as a potential immunomodulator in feline injection-site sarcoma: a hypothesis on tumor-macrophage interactions.

The Veterinary Quarterly · August 2, 2026

Hypothesis-Generating Why this matters to you Expert discussion

Raises a question worth testing. It does not answer one.

AI Summary

This is a narrative review proposing a hypothesis that feline AIM, through its distinctive high-affinity IgM binding and macrophage-derived immunomodulatory functions, may shift the tumor microenvironment toward M2 macrophage polarization and immunosuppression in feline injection-site sarcoma. The hypothesis is mechanistic and exploratory, grounded in known AIM biology in other species and FISS pathology, but contains no experimental validation or clinical evidence.

What was studied

Journal article. Cats with feline injection-site sarcoma (FISS); macrophage biology in mammals (primarily reviewed in mice and humans as experimental models).

Key findings

Feline AIM exists as two structural variants (37 kDa and 45 kDa) generated through exon 3 duplication, distinct from human/mouse AIM Feline AIM binds IgM approximately 1000-fold more tightly than murine AIM due to a species-specific positively charged cluster in the third SRCR domain TAM density in FISS correlates with tumor aggressiveness; higher TAM infiltration associated with increased necrosis, mitotic index, and poorer differentiation

Safety

High-affinity IgM binding of feline AIM may constrain free AIM availability in vivo, shifting functional balance away from complement-mediated cytotoxicity toward M2-polarizing pathways

Who this may help

This hypothesis does not support clinical action. It identifies AIM as a candidate for future mechanistic investigation in feline tumor immunology and may guide laboratory research priorities, but empirical studies in FISS are needed to test whether AIM modulation influences outcomes.

Plain-language summary

This is a narrative review proposing a theoretical model of AIM's potential role in feline injection-site sarcoma, with no empirical data, experimental evidence, or clinical outcomes reported.

Reported figures

Quoted from the source exactly as published.

Feline AIM IgM binding affinity relative to mouseapproximately 1000-fold higher

Feline AIM structural variants37 kDa and 45 kDa forms

Human serum AIM concentration≈5 µg/mL

Clinical bottom line

Caveats

No primary data presented; all evidence is synthesized from prior literature or derived from theoretical reasoning
No experimental evidence that AIM influences FISS development, progression, or response to therapy in cats
Mechanistic assumptions rely on murine HCC and general AIM biology; direct feline FISS experiments not conducted
High-affinity feline AIM-IgM binding is described as a property but its functional consequences in the tumor microenvironment are speculative

Evidence assessment

Graded across the dimensions that decide whether you should act, each from what the source actually supports. There is no single score, and where a dimension was not assessed it says so.

Study design

insufficient

Narrative review; no primary data collection or experimental design reported.

Directness

limited

Proposes theoretical mechanism; no direct evidence linking AIM to FISS outcomes.

Consistency

insufficient

No comparative evidence or conflicting data presented; primarily literature synthesis.

Risk of bias

insufficient

Not applicable to hypothesis-generating review; no outcome assessment.

Sample adequacy

insufficient

No empirical sample; no study population enrolled or analyzed.

Source abstract

AIM (apoptosis inhibitor of macrophage), also known as CD5L, is a macrophage-derived scavenger protein with broad immunomodulatory roles in mammals. Secreted by tissue macrophages, AIM circulates bound to IgM pentamers, which stabilize the protein in blood; free AIM is released during inflammation to influence immune signaling and cellular homeostasis. (Yang et al. 2023). In cats, AIM exhibits distinctive structural and biochemical properties, existing as both a three-domain (37 kDa) and a four-domain (45 kDa) variant generated through exon 3 duplication (Evangelista et al. 2025), and binding IgM approximately 1000-fold more tightly than murine AIM due to a species-specific positively charged cluster in its third SRCR domain (Miyazaki et al., 2018). These differences suggest feline AIM may function distinctly from human/mouse AIM. Feline injection-site sarcomas (FISS) arise in the context of chronic inflammation and frequently contain abundant tumor-associated macrophages (TAMs)associated with tumor aggressiveness and progression (Gomes et al., 2025). Given AIM's macrophage origin and its established roles in inflammatory regulation, autophagy signaling, and complement-mediated tumor targeting, we propose a directional hypothesis: that the high-affinity IgM-binding of feline AIM constrains free AIM availability in vivo, shifting the functional balance away from complement-mediated tumor cytotoxicity and toward macrophage-intrinsic M2-polarizing pathways, thereby promoting an immunosuppressive tumor microenvironment that contributes to FISS progression. Exploring AIM in this context may provide new perspectives for investigation in feline tumor immunology.

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Míra MándokiInflammation Driven CancerTumor Associated MacrophagesAnna SzilasiTumor MicroenvironmentGyula BalkaCat DiseasesApoptosis Inhibitor of Macrophage (aim/cd5l)Julia GilbertReceptors, Scavenger