Lessons from NK Cell Deficiencies in the Mouse

Type 1 Innate Lymphoid Cells Limit the Antitumoral Immune Response

Natural killer (NK) cells are known to be able to kill established tumor cell lines, but important caveats remain regarding their roles in the detection and elimination of developing primary tumors. Using a genetic model of selective ILC1 and NK cell deficiency, we showed that these cells were dispensable for tumor immunosurveillance and immunoediting in the MCA-induced carcinogenesis model. However, we were able to generate primary cell lines derived from MCA-induced tumors with graded sensitivity to NK1.1⁺ cells (including NK cells and ILC1). This differential sensitivity was associated neither with a modulation of intratumoral NK cell frequency, nor the capacity of tumor cells to activate NK cells. Instead, ILC1 infiltration into the tumor was found to be a critical determinant of NK1.1⁺ cell-dependent tumor growth. Finally, bulk tumor RNAseq analysis identified a gene expression signature associated with tumor sensitivity to NK1.1⁺ cells. ILC1 therefore appear to play an active role in inhibiting the antitumoral immune response, prompting to evaluate the differential tumor infiltration of ILC1 and NK cells in patients to optimize the harnessing of immunity in cancer therapies.

Affinity Maturation Is Impaired by Natural Killer Cell Suppression of Germinal Centers

Somatic hypermutation of immunoglobulin sequences in germinal center (GC) reactions must be optimized to elicit high-affinity, protective antibodies after vaccination. We expose natural killer (NK) cells as robust negative regulators of somatic hypermutation in antigen-reactive B cells. NK cells restrict follicular helper T cell (T_FH) and GC B cell frequencies and titers of antigen-specific immunoglobulin after administration of alum-adjuvanted hapten-protein conjugate vaccines. This inhibition is perforin dependent, suggesting that NK cells kill one or more cells critical for GC development. In the presence of perforin-competent NK cells, antigen-specific GC B cells acquire fewer mutations, including less frequent generation of non-synonymous substitutions and mutations associated with increased antibody affinity. Thus, NK cells limit the magnitude of GC reactions and thereby restrain vaccine elicitation of high-affinity antibodies. Circumventing this activity of NK cells during vaccination has strong potential to enhance humoral immunity and facilitate vaccine-elicited prevention of disease.

Role of NKp46+ natural killer cells in house dust mite-driven asthma

House dust mite (HDM)‐allergic asthma is driven by T helper 2 (Th2) lymphocytes, but also innate immune cells control key aspects of the disease. The precise function of innate natural killer (NK) cells during the initiation and propagation of asthma has been very confusing, in part because different, not entirely specific, strategies were used to target these cells. We show that HDM inhalation rapidly led to the accumulation of NK cells in the lung‐draining lymph nodes and of activated CD69⁺ NK cells in the bronchoalveolar lumen. However, genetically engineered Ncr1‐DTA or Ncr1‐DTR mice that constitutively or temporarily lack NK cells, still developed all key features of acute or chronic HDM‐driven asthma, such as bronchial hyperreactivity, Th2 cytokine production, eosinophilia, mucus overproduction, and Th2‐dependent immunoglobulin serum titers. The same results were obtained by administration of conventional NK1.1 or asialo‐GM1 NK cell‐depleting antibodies, antibody‐mediated blocking of the NKG2D receptor, or genetic NKG2D deficiency. Thus, although NK cells accumulate in allergen‐challenged lungs, our findings comprehensively demonstrate that these cells are not required for HDM‐driven asthma in the mouse.

Complement factor P is a ligand for the natural killer cell-activating receptor NKp46

Innate lymphoid cells (ILCs) are involved in immune responses to microbes and various stressed cells, such as tumor cells. They include group 1 [such as natural killer (NK) cells and ILC1], group 2, and group 3 ILCs. Besides their capacity to respond to cytokines, ILCs detect their targets through a series of cell surface-activating receptors recognizing microbial and nonmicrobial ligands. The nature of some of these ligands remains unclear, limiting our understanding of ILC biology. We focused on NKp46, which is highly conserved in mammals and expressed by all mature NK cells and subsets of ILC1 and ILC3. We show here that NKp46 binds to a soluble plasma glycoprotein, the complement factor P (CFP; properdin), the only known positive regulator of the alternative complement pathway. Consistent with the selective predisposition of patients lacking CFP to lethal Neisseria meningitidis (Nm) infections, NKp46 and group 1 ILCs bearing this receptor were found to be required for mice to survive Nm infection. Moreover, the beneficial effects of CFP treatment for Nm infection were dependent on NKp46 and group 1 NKp46+ ILCs. Thus, group 1 NKp46+ ILCs interact with the complement pathway, via NKp46, revealing a cross-talk between two partners of innate immunity in the response to an invasive bacterial infection.