# Intracellular Recognition of Fungi by the Innate Immune System

> **NIH NIH R21** · UNIVERSITY OF CALIFORNIA, SAN FRANCISCO · 2020 · $241,500

## Abstract

Fungal infections remain among the most difficult to treat. Innate recognition of microbes is critical for effective
host responses, and is often accomplished by myeloid cells including macrophages. Bacterial PAMPs are
recognized by cell surface, endosomal, and cytoplasmic receptors. Cytoplasmic recognition of bacterial PAMPS
has been suggested to enable the innate immune system to distinguish pathogens from noninvasive
commensals. The fungal PAMPs glucan and mannan are recognized by cell surface C-type lectin receptors,
but no cytoplasmic receptors have been identified for fungal-specific PAMPs. Early studies of intracellular
detection of bacterial cell wall components utilized direct cytoplasmic introduction of PAMPs such as
lipopolysaccharide (LPS) to infer the existence of cytoplasmic receptors. Depending on the PAMP, cytoplasmic
introduction of such components can induce a Nuclear Factor kappa B (NF-kB) response (via the NOD1 or NOD2
receptors) and/or the activation of inflammasomes. We employed this cytoplasmic introduction approach using
material extracted from the fungal cell wall preparation zymosan (“soluble zymosan”) and found that it can
potently trigger an NF-kB response. Further, we have found that this response—which requires the cytoplasmic
introduction of soluble zymosan—is genetically independent of the Toll-like receptor (TLR) signaling adaptor
MyD88 and the C-type lectin (CLR) signaling components Syk and CARD9. Together, these data argue that
known cell surface sensing mechanisms do not mediate this effect. Partial purification of the activity has identified
a protease-sensitive soluble fraction that displays dose-dependent NF-kB responses without activating
inflammasome responses (IL-1 secretion). As LPS does not have these properties upon transfection (LPS
activates both NF-kB and IL-1 responses and is not protease-sensitive), these and other experiments strongly
argue against the possibility that LPS contamination of zymosan is the source of this activity. For brevity, we
refer to this activity as the CSZR (cytoplasmic soluble zymosan response). Ultimate proof of its existence and
importance requires the molecular identification of the relevant receptor and microbial inducer. In this high-risk,
exploratory R21 proposal, we will use classical biochemical purification and mass spectrometry to purify and
identify the inducing ligand and we will perform a whole-genome CRISPR screen in immortalized bone marrow-
derived macrophages to identify candidates for the cognate ligand receptor. Significance/potential impact: This
work has the potential to reveal a previously unrecognized intracellular sensing mechanism for a fungal PAMP
that has the potential to transform our understanding of innate immunity to fungi. Such knowledge may yield new
approaches for the effective treatment of invasive fungal disease, an increasing global threat to human health.

## Key facts

- **NIH application ID:** 9996287
- **Project number:** 1R21AI150083-01A1
- **Recipient organization:** UNIVERSITY OF CALIFORNIA, SAN FRANCISCO
- **Principal Investigator:** Hiten D Madhani
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $241,500
- **Award type:** 1
- **Project period:** 2020-01-22 → 2021-12-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9996287

## Citation

> US National Institutes of Health, RePORTER application 9996287, Intracellular Recognition of Fungi by the Innate Immune System (1R21AI150083-01A1). Retrieved via AI Analytics 2026-05-25 from https://api.ai-analytics.org/grant/nih/9996287. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*